qemu: fix build with clang

qemuMigrationResetTLS() does not initialize 'ret' by default,
so when it jumps to 'cleanup' on error, the 'ret' variable will be
uninitialized, which clang complains about.

Set it to '-1' by default.

src/qemu/qemu_migration.c

@@ -3815,7 +3815,7 @@ qemuMigrationResetTLS(virQEMUDriverPtr driver,
     char *tlsAlias = in_tlsAlias;
     char *secAlias = in_secAlias;
     qemuMonitorMigrationParams migParams = { 0 };
-    int ret;
+    int ret = -1;
 
     /* If coming from a path that doesn't know whether it's been used or not,
      * let's first check we need to do this. If the tls-creds doesn't exist