block/raw-posix: bdrv_parse_filename() for floppy

The "host_floppy" protocol driver should strip the "host_floppy:" 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>