vdsm: pre-defined range for spice/vnc ports

Libvirt's default port range 5900-65535 which is out of VDSM's
documented range 5634 to 6166. But since the medieval time we
already use libvirt's default. We'll define the new range as
5900-6923 (1024 ports) So:

1. Set some limited port range, that can be used in FW conf
2. Use start port 5900 as it is already used by customers
3. Use the endport as 6923
4. Change VDSM documentation according to new port range

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1052162
Signed-off-by: Peter V. Saveliev <peet@redhat.com>
Signed-off-by: Vinzenz Feenstra <vfeenstr@redhat.com>
Change-Id: Id42d41064e122f1b2dcc98506b58d0cc74239650
Reviewed-on: http://gerrit.ovirt.org/17058
Reviewed-by: Michal Skrivanek <michal.skrivanek@redhat.com>
Tested-by: Michal Skrivanek <michal.skrivanek@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/23940
Reviewed-by: Yaniv Bronhaim <ybronhei@redhat.com>