raw-posix: add a raw_open_common helper

raw_open and hdev_open contain the same basic logic.  Add a new
raw_open_common helper containing the guts of the open routine
and call it from raw_open and hdev_open.

We use the new open_flags field in BDRVRawState to allow passing
additional open flags to raw_open_common from both.

Signed-off-by: Christoph Hellwig <hch@lst.de>