bhyve virtio-net: Allow backend type to be explicitly specified.

Surrently virtio-net uses the prefix of the backing interface to
choose the backend. This patch adds an additional option "type" to
choose the backend type explicitly. This allows greater flexibility
for end users to manage bhyve specific resources (such as by naming
the tap interfaces to more descriptive names). The option "type" is
optional. When it is not presented, the backend is derived from the
name of the backend interface.

For example, the line -s 3,virtio-net,bsdvm0,type=tap will create a
virtio-net device for the guest using the tap interface "bsdvm0".

Adding a new "type" option preserves the current legacy format in which
the first value after virtio-net names an instance of a backend.

Note that tap interfaces not following the pattern "tap*" will not be
created on demand via devfs cloning but must be created explicitly.

Reviewed by: vmaffione, jhb
Differential Revision: https://reviews.freebsd.org/D35143