virt: add get_instance_disk_info to virt driver API

The compute manager has been calling get_instance_disk_info on
virt driver implementations during block migration, despite
this method not being declared in the virt driver API. It was
implemented in libvirt and stubbed in xenapi, with different
API signatures in both.

Declare & document it as part of the virt driver API, and
split the libvirt impl into two parts to hide the libvirt
specific notion of XML config.

Partial-bug: #1333219
Change-Id: I38c811f91cce99772ff988a278c32483a219949d
Merge "virt: add get_instance_disk_info to virt driver API"