Packaging: fix ./configure option for libvirt environment file

The help string "=FILE" is incorrectly added to the name of the
configure option --with-libvirt-service-default. So This patch moves
"=FILE" to help string.

Change-Id: I34fa3764b999e71186171931664b0481b2ef3a15
Signed-off-by: Zhou Zheng Sheng <zhshzhou@linux.vnet.ibm.com>
Reviewed-on: http://gerrit.ovirt.org/20168
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Tested-by: Dan Kenigsberg <danken@redhat.com>