raw-posix: open flags use BDRV_ namespace, not posix namespace

The flags argument to raw_common_open() contain bits defined by the BDRV_O_*
namespace, not the posix O_* namespace.

Adjust to use the correct constants.

Signed-off-by: Avi Kivity <avi@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>