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 ddf8ad8, useless code
introduced by f03dcc5.

https://bugzilla.redhat.com/show_bug.cgi?id=977961
(cherry picked from commit 5744d96f211160d406ec0498c2f814a67d1a3fc8)

src/qemu/qemu_migration.c

@@ -2596,8 +2596,6 @@ 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) {