tools: 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: Ie5521021fc6d826e8f70706e4eb7e971339e8e56
Signed-off-by: Tomas Jelinek <tjelinek@redhat.com>