Merge remote-tracking branch 'remotes/cschoenebeck/tags/pull-9p-20200915' into staging

Merge remote-tracking branch 'remotes/cschoenebeck/tags/pull-9p-20200915' into staging

The intention of the following two patches is making users aware about
the negative file I/O performance impact when using a very low value
for 9P client parameter 'msize', which especially is the case if no
'msize' parameter was supplied by the user with a 9P Linux client at all.

All it does is logging a performance warning on host side (once) in
that case. By setting 'msize' on client side to any value larger than
8192 the performance warning will disappear.

See https://wiki.qemu.org/Documentation/9psetup#msize for details.

# gpg: Signature made Tue 15 Sep 2020 11:37:32 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-20200915:
  9pfs: disable msize warning for synth driver
  9pfs: log warning if msize <= 8192

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