core: check QEMU compatibility on livesnapshot

This patch disables live snapshotting if the
VDS currently running a VM reports QEMU does not
supports it.

Change-Id: Id8869bab587d728b67d9faa0f72fe5a3a9e8ac87
Bug-Url: https://bugzilla.redhat.com/1009100
Signed-off-by: Francesco Romani <fromani@redhat.com>