Expose the amount of resident and wired memory from the guest's vmspace.

This is different than the amount shown for the process e.g. by
/usr/bin/top - that is the mappings faulted in by the mmap'd region
of guest memory.

The values can be fetched with bhyvectl

 # bhyvectl --get-stats --vm=myvm
 ...
 Resident memory                         	413749248
 Wired memory                            	0
 ...

vmm_stat.[ch] -
 Modify the counter code in bhyve to allow direct setting of a counter
as opposed to incrementing, and providing a callback to fetch a
counter's value.

Reviewed by:	neel