qemu: Ignore subsequent attempts to probe device properties

In some cases, we are probing multiple devices for the same
property and setting the corresponding capability if it's
found on any of the devices: when that happens, we can quit
early after finding the first property and avoiding a bunch
of string comparisons.
Signed-off-by: NAndrea Bolognani <abologna@redhat.com>

src/qemu/qemu_capabilities.c

@@ -1978,6 +1978,9 @@ virQEMUCapsProcessStringFlags(virQEMUCapsPtr qemuCaps,
 {
     size_t i, j;
     for (i = 0; i < nflags; i++) {
+        if (virQEMUCapsGet(qemuCaps, flags[i].flag))
+            continue;
+
         for (j = 0; j < nvalues; j++) {
             if (STREQ(values[j], flags[i].value)) {
                 virQEMUCapsSet(qemuCaps, flags[i].flag);