qemu-nbd: only send a limited number of errno codes on the wire

Right now, NBD includes potentially platform-specific error values in
the wire protocol.

Luckily, most common error values are more or less universal: in
particular, of all errno values <= 34 (up to ERANGE), they are all the
same on supported platforms except for 11 (which is EAGAIN on Windows and
Linux, but EDEADLK on Darwin and the *BSDs).  So, in order to guarantee
some portability, only keep a handful of possible error codes and squash
everything else to EINVAL.

This patch defines a limited set of errno values that are valid for the
NBD protocol, and specifies recommendations for what error to return
in specific corner cases.  The set of errno values is roughly based on
the errors listed in the read(2) and write(2) man pages, with some
exceptions:

- ENOMEM is added for servers that implement copy-on-write or other
  formats that require dynamic allocation.

- EDQUOT is not part of the universal set of errors; it can be changed
  to ENOSPC on the wire format.

- EFBIG is part of the universal set of errors, but it is also changed
  to ENOSPC because it is pretty similar to ENOSPC or EDQUOT.

Incoming values will in general match system errno values, but not
on the Hurd which has different errno values (they have a "subsystem
code" equal to 0x10 in bits 24-31).  The Hurd is probably not something
to which QEMU has been ported, but still do the right thing and
reverse-map the NBD errno values to the system errno values.

The corresponding patch to the NBD protocol description can be found at
http://article.gmane.org/gmane.linux.drivers.nbd.general/3154.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
