core: Added SpiceProxyDefault property to vdc_options

The value of SpiceProxyDefault is in form of  [protocol://]<host>[:port]
and will be used in FE to fill into the spice client

Change-Id: I0692bfc29765bee4839b35b658ee388b95728d2f
Signed-off-by: Tomas Jelinek <tjelinek@redhat.com>