vnc: Fix fatal crash with vnc reverse mode

Reverse mode is unusable:

	qemu -vnc localhost:5500,reverse

crashes in vnc_refresh_server_surface because some pointers are NULL.

Fix this by calling vnc_dpy_resize (which initializes these pointers)
before calling vnc_refresh.

Cc: Anthony Liguori <aliguori@us.ibm.com>
Signed-off-by: Stefan Weil <weil@mail.berlios.de>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
