qemu: don't use initialized ret in qemuRemoveSharedDevice

This fixes

  CC     qemu/libvirt_driver_qemu_impl_la-qemu_conf.lo
  qemu/qemu_conf.c: In function 'qemuRemoveSharedDevice':
  qemu/qemu_conf.c:1384:9: error: 'ret' may be used uninitialized in this function [-Werror=maybe-uninitialized]

src/qemu/qemu_conf.c

@@ -1381,7 +1381,7 @@ qemuRemoveSharedHostdev(virQEMUDriverPtr driver,
 {
     char *dev_path = NULL;
     char *key = NULL;
-    int ret;
+    int ret = -1;
 
     if (!qemuIsSharedHostdev(hostdev))
         return 0;