Make qemu-common obsolete RBD module when disabled
If RBD module build is disabled, we need to obsolete it to ensure a
clean upgrade from previous builds. This is done by adding an Obsoletes
from the qemu-common sub-RPM.
This fixes the upgrade path on i686 (rhbz #1688117)
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>