qemu: detect multi-head qxl via more than version check

qemu: detect multi-head qxl via more than version check

Multi-head QXL support is so useful that distros have started to
backport it to qemu earlier than 1.2.  After discussion with
Alon Levy, we determined that the existence of the qxl-vga.surfaces
property is a reliable indicator of whether '-device qxl-vga' works,
or whether we have to stick to the older '-vga qxl'.  I'm leaving
in the existing check for QEMU_CAPS_DEVICE_VIDEO_PRIMARY tied to
qemu 1.2 and newer (in case qemu is built without qxl support),
but for those distros that backport qxl, this additional capability
check will allow the correct command line for both RHEL 6.3 (which
lacks the feature) and RHEL 6.4 (where qemu still claims to be
version 0.12.2.x, but has backported multi-head qxl).

* src/qemu/qemu_capabilities.c (virQEMUCapsObjectPropsQxlVga): New
property test.
(virQEMUCapsExtractDeviceStr): Probe for backport of new
capability to qemu earlier than 1.2.
* tests/qemuhelpdata/qemu-kvm-1.2.0-device: Update test.
* tests/qemuhelpdata/qemu-1.2.0-device: Likewise.
* tests/qemuhelpdata/qemu-kvm-0.12.1.2-rhel62-beta-device:
Likewise.