block/raw-posix: bdrv_parse_filename() for hdev

The "host_device" protocol driver should strip the "host_device:" prefix
from filenames if present.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Benoit Canet <benoit@irqsave.net>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>