spec: use Requires %{name}

We should use macros instead of static names in the spec.
If in the future we decide to use different name for vdsm package
it's a matter to put the new package name in Name session.

Change-Id: I6a20093e59990127598d70918b62ab9ea3d5c429
Signed-off-by: Douglas Schilling Landgraf <dougsland@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/24301
Reviewed-by: Kiril Nesenko <knesenko@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>