virtio-net: Fix lduw_p() pointer argument of wrong size

A pointer to a size_t variable was passed as the void * pointer to
lduw_p() in virtio_net_receive().  Instead of acting on the 16-bit value
this caused failure on big-endian hosts.

Avoid this issue in the future by using stw_p() instead.  In general we
should use ld*_p() for loading from target memory and st*_p() for
storing to target memory anyway, not the other way around.

Also tighten up a correct use of lduw_p() when stw_p() should be used
instead in virtio_net_get_config().

Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>