Add common config options for SPICE graphics

The VNC config options are shared across hypervisor drivers, and
while SPICE originally only supported KVM, it is now possible to
use it with Xen too. Thus, adding common SPICE config options
makes more sense than adding them to the libvirt driver only

DocImpact
Blueprint: libvirt-spice
Change-Id: I8e817530908c6fee0781b92dc9e27010951b4540
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Merge "Add common config options for SPICE graphics"