Implement QEMU/KVM QXL video card support in QEMU driver

Implement QEMU/KVM QXL video card support in QEMU driver

This supports the '-vga qxl' parameter in upstream QEMU/KVM
which has SPICE support added. This isn't particularly useful
until you get the next patch for -spice support. Also note that
while the libvirt XML supports multiple video devices, this
patch only supports a single one. A later patch can add support
for 2nd, 3rd, etc PCI devices for QXL

* src/qemu/qemu_conf.h: Flag for QXL support
* src/qemu/qemu_conf.c: Probe for '-vga qxl' support and implement it
* tests/qemuxml2argvtest.c, tests/qemuxml2xmltest.c,
  tests/qemuxml2argvdata/qemuxml2argv-graphics-spice.args,
  tests/qemuxml2argvdata/qemuxml2argv-graphics-spice.xml: Test
  case for generating spice args with RHEL6 kvm