hw/9pfs: Fix errno value for xattr functions

If there is no operation driver for the xattr type the
functions return '-1' and set errno to '-EOPNOTSUPP'.
When the calling code sets 'ret = -errno' this turns
into a large positive number.

In Linux 3.11, the kernel has switched to using 9p
version 9p2000.L, instead of 9p2000.u, which enables
support for xattr operations. This on its own is harmless,
but for another change which makes it request the xattr
with a name 'security.capability'.

The result is that the guest sees a succesful return
of 95 bytes of data, instead of a failure with errno
set to 95. Since the kernel expects a maximum of 20
bytes for an xattr return this gets translated to the
unexpected errno ERANGE.

This all means that when running a binary off a 9p fs
in 3.11 kernels you get a fun result of:

  # ./date
  sh: ./date: Numerical result out of range

The only workaround is to pass 'version=9p2000.u' when
mounting the 9p fs in the guest, to disable all use of
xattrs.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Reviewed-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
