raw-posix: fix hdev_create

We do need hdev_create unconditionally on all platforms so that qemu-img
create support for host device works on all platforms.

Also relax the check to allow character devices in addition to block
devices.  On many Unix platforms block devices have buffered block
nodes and unbuffered character device nodes, and on FreeBSD the block
nodes don't even exist anymore.  Also on Linux we do support the
/dev/sgN scsi passthrough devices through the host device driver,
and probably the old-style /dev/raw/rawN raw devices although I haven't
tested that.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>