Improve shared storage checks for live migration

Distinguish cases that require shared instance drive and shared libvirt
instance directory. Reflect the fact that RBD backed instances have
shared instance drive (and no shared libvirt instance directory) in the
relevant conditionals.

Change-Id: I2755c59b4db736151000dae351fd776d3c15ca39
Signed-off-by: Dmitry Borodaenko <dborodaenko@mirantis.com>