linux-user: add struct old_dev_t compat

The compat LOOP_SET_STATUS ioctl uses struct old_dev_t in its passed
struct. That variable type is vastly different between different
architectures. Implement wrapping around it so we can use it.

This fixes running arm kpartx on an x86_64 host for me.

Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>
