Replace centos macro for rhel

Koji system doesn't understand centos macro and to generate EL6 rpm
compatible on it we should replace the macro (there is no vhostmd
build for el6 upstream). On the other hand, in downstream there
is vhostmd package and we should keep that compatible with
downstream only patch.

Change-Id: I3a6a6086afec7d06df464f843e86679cb8b7b80e
Signed-off-by: Douglas Schilling Landgraf <dougsland@redhat.com>