qemu: replace multiple strcmps with a switch on an enum

I'm not sure why this code was written to compare the strings that it
had just retrieved from an enum->string conversion, rather than just
look at the original enum values, but this yields the same results,
and is much more efficient (especially as you add more devices).

This is a prerequisite for patches to resolve:

   https://bugzilla.redhat.com/show_bug.cgi?id=1003983

src/qemu/qemu_command.c

@@ -5269,13 +5269,18 @@ qemuBuildSoundDevStr(virDomainDefPtr def,
         goto error;
     }
 
-    /* Hack for weirdly unusual devices name in QEMU */
-    if (STREQ(model, "es1370"))
+    /* Hack for devices with different names in QEMU and libvirt */
+    switch (sound->model) {
+    case VIR_DOMAIN_SOUND_MODEL_ES1370:
         model = "ES1370";
-    else if (STREQ(model, "ac97"))
+        break;
+    case VIR_DOMAIN_SOUND_MODEL_AC97:
         model = "AC97";
-    else if (STREQ(model, "ich6"))
+        break;
+    case VIR_DOMAIN_SOUND_MODEL_ICH6:
         model = "intel-hda";
+        break;
+    }
 
     virBufferAsprintf(&buf, "%s,id=%s", model, sound->info.alias);
     if (qemuBuildDeviceAddressStr(&buf, def, &sound->info, qemuCaps) < 0)