Add a <graphics> type for SPICE protocol

Add a <graphics> type for SPICE protocol

This adds an element

 <graphics type='spice' port='5903' tlsPort='5904' autoport='yes' listen='127.0.0.1'/>

This is the bare minimum that should be exposed in the guest
config for SPICE. Other parameters are better handled as per
host level configuration tunables

* docs/schemas/domain.rng: Define the SPICE <graphics> schema
* src/domain_conf.h, src/domain_conf.c: Add parsing and formatting
  for SPICE graphics config
* src/qemu_conf.c: Complain about unsupported graphics types