vdsm.spec: Add with_vhostmd

The package vhostmd is only available on Fedora, the EL* version
is not available in EPEL.

Change-Id: Iffacf45abc2dfe866d4f3ea28e872a2fdddd69dc
Signed-off-by: Douglas Schilling Landgraf <dougsland@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/23126
Reviewed-by: Sandro Bonazzola <sbonazzo@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>