vdsm.spec: conditional to execute semanage

To avoid load policy when selinux is disabled, only
execute semanage when selinux is enabled.

Change-Id: I1d40e6d9dcaac16d74c685e4a6a6d5cb130ac531
Signed-off-by: Douglas Schilling Landgraf <dougsland@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/21830
Reviewed-by: Yaniv Bronhaim <ybronhei@redhat.com>
Tested-by: Yaniv Bronhaim <ybronhei@redhat.com>