Merge remote-tracking branch 'remotes/cschoenebeck/tags/pull-9p-20201019' into staging

Merge remote-tracking branch 'remotes/cschoenebeck/tags/pull-9p-20201019' into staging

9pfs: add tests using local fs driver

The currently existing 9pfs test cases are all solely using the 9pfs 'synth'
fileystem driver, which is a very simple and purely simulated (in RAM only)
filesystem. There are issues though where the 'synth' fs driver is not
sufficient. For example the following two bugs need test cases running the
9pfs 'local' fs driver:

https://bugs.launchpad.net/qemu/+bug/1336794
https://bugs.launchpad.net/qemu/+bug/1877384

This patch set for that reason introduces 9pfs test cases using the 9pfs
'local' filesystem driver along to the already existing tests on 'synth'.

# gpg: Signature made Mon 19 Oct 2020 13:39:08 BST
# gpg:                using RSA key 96D8D110CF7AF8084F88590134C2B58765A47395
# gpg:                issuer "qemu_oss@crudebyte.com"
# gpg: Good signature from "Christian Schoenebeck <qemu_oss@crudebyte.com>" [unknown]
# gpg: WARNING: This key is not certified with a trusted signature!
# gpg:          There is no indication that the signature belongs to the owner.
# Primary key fingerprint: ECAB 1A45 4014 1413 BA38  4926 30DB 47C3 A012 D5F4
#      Subkey fingerprint: 96D8 D110 CF7A F808 4F88  5901 34C2 B587 65A4 7395

* remotes/cschoenebeck/tags/pull-9p-20201019:
  tests/9pfs: add local Tmkdir test
  tests/9pfs: add virtio_9p_test_path()
  tests/9pfs: wipe local 9pfs test directory
  tests/9pfs: introduce local tests
  tests/9pfs: change qtest name prefix to synth
  9pfs: suppress performance warnings on qtest runs

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>