Add support for configuring SPICE graphics with libvirt

Add support to the libvirt driver to configure SPICE graphics
and the SPICE guest agent. It allows for enablement of both
VNC and SPICE at the same time, since some recent libvirt/KVM
versions support this concurrent mode.

The USB tablet will now only be enabled if VNC is requested,
or SPICE is requested without the agent

Blueprint: libvirt-spice
Change-Id: Ic7fbfd636455aba5bf881b6a1925fd4561edfd15
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Merge "Add support for configuring SPICE graphics with libvirt"