hw/9pfs: Don't return type from host in readdir on local 9p filesystem

When using mapped mode in 9pfs, readdir implementation
should not return file type in d_type from the host
readdir, instead, it should use the type stored in
the extended attributes.  Since d_type is optional
and reading ext attrs for every readdir is expensive,
it should be sufficient to just set d_type to DT_UNKNOWN,
so guest will know to look it up separately.

This is a -stable material.

Signed-off-by: Bastian Blank <waldi@debian.org>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>