Add bhyve support to virt-host-validate(1). It checks for the
essential kernel modules to be available so that user can actually
start VMs, have networking and console access.

It uses the kldnext(2)/kldstat(2) routines to retrieve modules list.
As bhyve is only available on FreeBSD and these routines were available
long before bhyve appeared, not adding any specific configure checks
for that.

Also, update tools/Makefile.am to add
virt-host-validate-$driver.[hc] to the build only if the
appropriate driver is enabled.