Implement QEMU/KVM support for SPICE graphics

Implement QEMU/KVM support for SPICE graphics

This supports the -spice argument posted for review against
the latest upstream QEMU/KVM. This supports the bare minimum
config with port, TLS port & listen address. The x509 bits are
added in a later patch.

* src/qemu_conf.c, src/qemu_conf.h: Add SPICE flag. Check for
  -spice availability. Format -spice arg for command line
* qemuhelptest.c: Add SPICE flag
* qemuxml2argvdata/qemuxml2argv-graphics-spice.args: Add <graphics>
  for spice
* qemuxml2argvdata/qemuxml2argv-graphics-spice.xml: Add -spice arg
* qemuxml2argvtest.c: Add SPICE flag