qemu: Detect VGA_QXL capability correctly

Since 4c993d8a we failed to set this important capability, which
allows starting a domain with QXL video card. We set DEVICE_QXL
capability bit instead, which is not necessary wrong. Anyway, if
qemu supports the new '-device qxl' it supports older '-vga qxl'
as well. The latter is used for the primary (the first) qxl video
card, the former for other video cards.

src/qemu/qemu_capabilities.c

@@ -2018,6 +2018,9 @@ qemuCapsProbeQMPObjects(qemuCapsPtr caps,
     /* Prefer -chardev spicevmc (detected earlier) over -device spicevmc */
     if (qemuCapsGet(caps, QEMU_CAPS_CHARDEV_SPICEVMC))
         qemuCapsClear(caps, QEMU_CAPS_DEVICE_SPICEVMC);
+    /* If qemu supports newer -device qxl it supports -vga qxl as well */
+    if (qemuCapsGet(caps, QEMU_CAPS_DEVICE_QXL))
+        qemuCapsSet(caps, QEMU_CAPS_VGA_QXL);
 
     return 0;
 }