guestrpc module to handle VMware backdoor port GuestRPC functionality

Convert existing FreeBSD vmware_hvcall function to take a channel
and parameter arguments.

Added vmware_guestrpc_cmd() to send GuestRPC commands to the VMware
hypervisor. The sbuf argument is used for both the command to send
and to store the data to return to the caller.

The following KPIs can be used to get and set FreeBSD-specific guest
information in key/value pairs:

• vmware_guestrpc_set_guestinfo
  • set a value into the guestinfo.fbsd.<keyword> key
• vmware_guestrpc_get_guestinfo
  • get the value stored in the guestinfo.fbsd.<keyword> key

Add VMware devices to x86 NOTES

Reviewed by: jhb
Obtained from: Juniper Networks, Inc.
Differential Revision: https://reviews.freebsd.org/D44528