Text: add description for dstqemut in migration command

Change-Id: I4cf7098537f3b2561a25bc77f4518b2a6ba0bfeb
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1007686
Signed-off-by: Mooli Tayer <mtayer@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/24009
Reviewed-by: Dan Kenigsberg <danken@redhat.com>