raw-posix.c: Make GetBSDPath() handle caching options

Add support for caching options that can be specified from the command
line.

The CD-ROM raw char device bypasses the host page cache and therefore
has alignment requirements.  Alignment probing is necessary so only use
the raw char device if BDRV_O_NOCACHE is set.

This patch fixes -cdrom /dev/cdrom on Mac OS X hosts, where bdrv_read()
used to fail due to misaligned requests during image format probing.

Signed-off-by: John Arbuckle <programmingkidx@gmail.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
