qemu: Always format formatStr for blockdev-snapshot-sync

The qemuDomainSnapshotPrepare should always set a > 0 format value anyway,
so remove the check.

Found by Coverity.

src/qemu/qemu_driver.c

@@ -14072,7 +14072,6 @@ qemuDomainSnapshotCreateSingleDiskActive(virQEMUDriverPtr driver,
     dd->prepared = true;
 
     /* create the actual snapshot */
-    if (dd->src->format)
     formatStr = virStorageFileFormatTypeToString(dd->src->format);
 
     /* The monitor is only accessed if qemu doesn't support transactions.