bhyve: Adjust method signatures to be in line with the ComputeDriver iface.

It is necessary after merging to the master branch, where the ComputeDriver
interface has changed a bit.
bhyve: New tap_name method of vif driver.
bhyve: New 'net_interfaces' property.
bhyve: Fix tap dev deletion while destroying already stoped VM.

It removes unnecessary attemts to delete a tap device of the stopped VM,
for which taps has already been deleted. This gets rid of warrnings
test: Add missing stub for isdir() function.

Previously, original isdir was called and result of the test depended on
the actual directory being present in the file system.