qemu: fix double free in qemuMigrationPrepareDirect

Remove assignment of the string freed by virURIFree
to hostname, since it's not used anywhere.

Double free introduced by ddf8ad82, useless code
introduced by f03dcc5d.

https://bugzilla.redhat.com/show_bug.cgi?id=977961

src/qemu/qemu_migration.c

@@ -2572,8 +2572,6 @@ qemuMigrationPrepareDirect(virQEMUDriverPtr driver,
             virReportError(VIR_ERR_INVALID_ARG, _("missing host in migration"
                                                   " URI: %s"), uri_in);
             goto cleanup;
-        } else {
-            hostname = uri->server;
         }
 
         if (uri->port == 0) {