vmm: add additional fault type, fixing vm receive

vmm: add additional fault type, fixing vm receive

After vmm(4)/vmd(8) "receive" a vm via vmctl(8), vmd on Intel hosts
was incorrectly identifying the fault type for nested page fault
exits and resetting the vm. This commit adds a new fault type to
signify the page fault was serviced in vmm and any exit to userland
does not require an assist.

ok mlarkin@