block/raw-posix: Don't think /dev/fd/<NN> is a floppy drive.

In libguestfs we use /dev/fd/<NN> to pass pre-opened file descriptors
to qemu-img.  Lately I've discovered that although this works, qemu
believes that these are floppy disk images.  That in itself isn't much
of a problem, but now qemu prints a warning about host floppy
pass-thru being deprecated.

Extend the existing test so that it ignores /dev/fd/ as well as
/dev/fdset/

A simple test of this, if you are using the bash shell, is:

  qemu-img info <( cat /dev/null )

without this patch:

  $ qemu-img info <( cat /dev/null )
  qemu-img: Host floppy pass-through is deprecated
  Support for it will be removed in a future release.
  qemu-img: Could not open '/dev/fd/63': Could not refresh total sector count: Illegal seek

with this patch:

  $ qemu-img info <( cat /dev/null )
  qemu-img: Could not open '/dev/fd/63': Could not refresh total sector count: Illegal seek

Signed-off-by: Richard W.M. Jones <rjones@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Message-id: 1435761614-31358-1-git-send-email-rjones@redhat.com
Fixes: https://bugs.launchpad.net/qemu/+bug/1470536
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
