mingw: Fix compilation breakage because of memmem use

mingw does not provide memmem. Since the qmp data is JSON, it won't
contain embedded nul characters, we can use strstr instead of
memmem.

Signed-off-by: Christophe Fergeau <cfergeau@redhat.com>
Acked-by: Frediano Ziglio <fziglio@redhat.com>
Acked-by: Marc-André Lureau <marcandre.lureau@gmail.com>

src/qmp-port.c

@@ -160,7 +160,7 @@ spice_qmp_handle_port_data(SpiceQmpPort *self, gpointer data,
     }
 
     str = qmp->str;
-    while ((crlf = memmem(str, qmp->len - (str - qmp->str), "\r\n", 2))) {
+    while ((crlf = strstr(str, "\r\n")) != NULL) {
         GError *err = NULL;
 
         *crlf = '\0';