core: lock VM for whole live disk migration process

In order to prevent a race between VM migration and storage commands,
the storage commands need to lock the VM for their entire execution.
This patch takes shared lock for the VM for the entire live storage
migration process.

Change-Id: I432e9230da697a1a67c5d40e926e16215c2583ef
Bug-Url: https://bugzilla.redhat.com/952147
Signed-off-by: Arik Hadas <ahadas@redhat.com>