Refactor configuration management in bhyve.

Replace the existing ad-hoc configuration via various global variables
with a small database of key-value pairs. The database supports
heirarchical keys using a MIB-like syntax to name the path to a given
key. Values are always stored as strings. The API used to manage
configuation values does include wrappers to handling boolean values.
Other values use non-string types require parsing by consumers.

The configuration values are stored in a tree using nvlists. Leaf
nodes hold string values. Configuration values are permitted to
reference other configuration values using '%(name)'. This permits
constructing template configurations.

All existing command line arguments now set configuration values. For
devices, the "-s" option parses its option argument to generate a list
of key-value pairs for the given device.

A new '-o' command line option permits setting an individual
configuration variable. The key name is always given as a full path
of dot-separated components.

A new '-k' command line option parses a simple configuration file.
This configuration file holds a flat list of 'key=value' lines where
the 'key' is the full path of a configuration variable. Lines
starting with a '#' are comments.

In general, bhyve starts by parsing command line options in sequence
and applying those settings to configuration values. Once this is
complete, bhyve then begins initializing its state based on the
configuration values. This means that subsequent configuration
options or files may override or supplement previously given settings.

A special 'config.dump' configuration value can be set to true to help
debug configuration issues. When this value is set, bhyve will print
out the configuration variables as a flat list of 'key=value' lines.

Most command line argments map to a single configuration variable,
e.g. '-w' sets the 'x86.strictmsr' value to false. A few command
line arguments have less obvious effects:

• Multiple '-p' options append their values (as a comma-seperated list) to "vcpu.N.cpuset" values (where N is a decimal vcpu number).

• For '-s' options, a pci.<bus>.<slot>.<function> node is created. The first argument to '-s' (the device type) is used as the value of a "device" variable. Additional comma-separated arguments are then parsed into 'key=value' pairs and used to set additional variables under the device node. A PCI device emulation driver can provide its own hook to override the parsing of the additonal '-s' arguments after the device type.
  
  After the configuration phase as completed, the init_pci hook then walks the "pci.<bus>.<slot>.<func>" nodes. It uses the "device" value to find the device model to use. The device model's init routine is passed a reference to its nvlist node in the configuration tree which it can query for specific variables.
  
  The result is that a lot of the string parsing is removed from the device models and centralized. In addition, adding a new variable just requires teaching the model to look for the new variable.

• For '-l' options, a similar model is used where the string is parsed into values that are later read during initialization. One key note here is that the serial ports use the commonly used lowercase names from existing documentation and examples (e.g. "lpc.com1") instead of the uppercase names previously used internally in bhyve.

Reviewed by: grehan
MFC after: 3 months
Differential Revision: https://reviews.freebsd.org/D26035