qemu: Fix QMP detection of QXL graphics

With the recent introduction of QMP capabilities probing, libvirt failed
to detect support for QXL graphics in QEMU 1.2 and newer. In addition to
fixing that, this patch also causes libvirt to detect QXL support for
qemu-kvm-0.13.0, which doesn't advertise it in -help output but mentions
it in device list. Since qemu-kvm-0.13.0 supported -spice, it looks like
not having qxl in -help was a bug.

src/qemu/qemu_capabilities.c

@@ -1271,6 +1271,7 @@ struct qemuCapsStringFlags qemuCapsObjectTypes[] = {
     { "virtio-scsi-pci", QEMU_CAPS_VIRTIO_SCSI_PCI },
     { "spicevmc", QEMU_CAPS_DEVICE_SPICEVMC },
     { "qxl-vga", QEMU_CAPS_DEVICE_QXL_VGA },
+    { "qxl", QEMU_CAPS_VGA_QXL },
     { "sga", QEMU_CAPS_SGA },
     { "scsi-block", QEMU_CAPS_SCSI_BLOCK },
     { "scsi-cd", QEMU_CAPS_SCSI_CD },

tests/qemuhelptest.c

@@ -467,6 +467,7 @@ mymain(void)
             QEMU_CAPS_NESTING,
             QEMU_CAPS_NAME_PROCESS,
             QEMU_CAPS_SMBIOS_TYPE,
+            QEMU_CAPS_VGA_QXL,
             QEMU_CAPS_SPICE,
             QEMU_CAPS_VGA_NONE,
             QEMU_CAPS_MIGRATE_QEMU_FD,