openvzDomainMigratePrepare3Params: correctly use hostname

In case no uri_in was supplied, we forgot to set the hostname
to the current hostname and formatted a useless uri_out.

src/util/glibcompat.h:57:26: error: ‘%s’ directive argument is null [-Werror=format-overflow=]
  57 | # define g_strdup_printf vir_g_strdup_printf
src/openvz/openvz_driver.c:2136:16: note: in expansion of macro ‘g_strdup_printf’
2136 |     *uri_out = g_strdup_printf("ssh://%s", hostname);

Signed-off-by: Ján Tomko <jtomko@redhat.com>
Reported-by: Jaroslav Suchanek <jsuchane@redhat.com>
Fixes: e3c626a61d6c3d808555653684c0fb1e7c4d74ec
Reviewed-by: Erik Skultety <eskultet@redhat.com>

src/openvz/openvz_driver.c

@@ -2113,6 +2113,8 @@ openvzDomainMigratePrepare3Params(virConnectPtr dconn,
                              " but migration requires an FQDN"));
             goto error;
         }
+
+        hostname = my_hostname;
     } else {
         uri = virURIParse(uri_in);