9p: remove support for the "handle" backend

The "handle" fsdev backend was deprecated in QEMU 2.12.0 with:

commit db3b3c72
Author: Greg Kurz <groug@kaod.org>
Date:   Mon Jan 8 11:18:23 2018 +0100

    9pfs: deprecate handle backend

    This backend raise some concerns:

    - doesn't support symlinks
    - fails +100 tests in the PJD POSIX file system test suite [1]
    - requires the QEMU process to run with the CAP_DAC_READ_SEARCH
      capability, which isn't recommended for security reasons

    This backend should not be used and wil be removed. The 'local'
    backend is the recommended alternative.

    [1] https://www.tuxera.com/community/posix-test-suite/



Signed-off-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Daniel P. Berrange <berrange@redhat.com>
Reviewed-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>

It has passed the two release cooling period without any complaint.

Remove it now.

Signed-off-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>