Simple sysctl-like firmware query interface. Similar in operation

Simple sysctl-like firmware query interface. Similar in operation
to the qemu one, and uses the same i/o ports but with different
messaging. Requires the 'bootrom' option to be enabled.

This is used by UEFI (and potentially other BIOSs/firmware) to
request information from bhyve. Currently, only the number of
vCPUs is made available, with more to follow.

A very large thankyou to Ben Perrault who helped out testing
an earlier version of this, and bhyve/Windows in general.

Reviewed by:	tychon
Discussed with:	neel
Sponsored by:	Nahanni Systems