baremetal: Add missing method to volume driver

Added get_hypervisor_version(), which is called by underlying
LibvirtVolumeDrivers. Also added tests roughly checking if the
underlying drivers can be loaded and be called without error.

Partial-Bug: #1208707
Change-Id: Ida6b6800d1ef518c6e76d6b2ab4ac2f795a744ca
Merge "baremetal: Add missing method to volume driver"