9pfs: deprecate handle backend

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 <[email protected]>
Reviewed-by: Daniel P. Berrange <[email protected]>
Reviewed-by: Aneesh Kumar K.V <[email protected]>