block/raw-posix.c: Fix raw_getlength() on Mac OS X block devices

block/raw-posix.c: Fix raw_getlength() on Mac OS X block devices

This patch replaces the dummy code in raw_getlength() for block devices
on OS X, which always returned LLONG_MAX, with a real implementation
that returns the actual block device size.

Signed-off-by: John Arbuckle <programmingkidx@gmail.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Tested-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>