rootwrap: Add commands needed by bhyve to compute fileters.
bhyve: Fix destroy method.

If there is no VM running /dev/vmm doesn't exist. This commit adjust bhyve
module to take this into account.
bhyve: Add function removing instance image file.
bhyve: Adjust the fake-based bhyve driver to actually spawn/destroy VMs.

This is bare mimimum necessary for succesfull spawn/destroy using nova
client. There is also a known issue, that VM has to be destroyed twice
in order to be successfully wiped out from nova database. The issue is
being investigated.