Add to libvirt driver instance_on_disk method

Added method to driver layer to validate that instance directory
accessible from the host.

part of libvirt driver support for blueprint rebuild-for-ha

DocImpact

Change-Id: I9c6a3aec99f61bd4fb6bc4b4264771e077aae96a
Co-authored-by: Oshrit Feder <oshritf@il.ibm.com>
Merge "Add to libvirt driver instance_on_disk method"