core: allow snapshots for read-only disks

Allow snapshots for read-only disks
(modified 'DiskImageBase -> isAllowSnapshot' accordingly).

Change-Id: Ic832544d2d520841abe1f20d0a9a724535481218
Bug-Url: https://bugzilla.redhat.com/1072471
Bug-Url: https://bugzilla.redhat.com/1072529
Signed-off-by: Daniel Erez <derez@redhat.com>