qemu_capabilities: join capabilities for qxl and qxl-vga devices

qemu_capabilities: join capabilities for qxl and qxl-vga devices

This patch simplifies QEMU capabilities for QXL video device.  QEMU
exposes this device as *qxl-vga* and *qxl* and they are both the same
device with the same set of parameters, the only difference is that
*qxl-vga* includes VGA compatibility.

Based on QEMU code they are tied together so it's safe to check only for
presence of only one of them.

This patch also removes an invalid test case "video-qxl-sec-nodevice"
where there is only *qxl-vga* device and *qxl* device is not present.

Signed-off-by: Pavel Hrdina <phrdina@redhat.com>