9pfs: code cleanup

* Use lock-guard design pattern instead of manual lock/unlock.

# gpg: Signature made Tue 16 Mar 2021 10:49:09 GMT
# 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-20210316:
  hw/9pfs/9p-synth: Replaced qemu_mutex_lock with QEMU_LOCK_GUARD

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