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.
Instead of embedding the pod information inside the respective
source files, store them in separate files.
This allows us to reduce the number of custom build rules as
most of the information can be inferred for the file name;
moreover, text editors are more likely to use proper syntax
highlighting for standalone pod files.
