If QEMU in question supports QMP, this capability is set if
QEMU_CAPS_DEVICE_QXL was set based on existence of "-device qxl". If
libvirt needs to parse *help*, because there is no QMP support, it
checks for existence of "-vga qxl", but it also parses output of
"-device ?" and sets QEMU_CAPS_DEVICE_QXL too.

Now that libvirt supports only QEMU that has "-device" implemented it's
safe to drop this capability and stop using it.
Signed-off-by: NPavel Hrdina <phrdina@redhat.com>