qemu -drive takes format= not fmt=

Seems like a simple typo - it has been "format=" since the flag
was introduced, but we added it as "fmt=".

Signed-off-by: Mark McLoughlin <markmc@redhat.com>

src/qemu_conf.c

@@ -1135,7 +1135,7 @@ int qemudBuildCommandLine(virConnectPtr conn,
                 disk->device == VIR_DOMAIN_DISK_DEVICE_DISK)
                 virBufferAddLit(&opt, ",boot=on");
             if (disk->driverType)
-                virBufferVSprintf(&opt, ",fmt=%s", disk->driverType);
+                virBufferVSprintf(&opt, ",format=%s", disk->driverType);
 
             if (disk->cachemode) {
                 const char *mode =