Merge remote-tracking branch 'remotes/gkurz/tags/9p-next-2020-05-26' into staging

- fix build with musl libc
- fix potential deadlock of QEMU main event loop (cannot be hit with linux
  client)
- revert 9pfs reply truncation (LP 1877688)
- xen backend waits for client to free space on the reply ring instead of
  truncating or disconnecting

# gpg: Signature made Tue 26 May 2020 10:36:23 BST
# gpg:                using RSA key B4828BAF943140CEF2A3491071D4D5E5822F73D6
# gpg: Good signature from "Greg Kurz <groug@kaod.org>" [full]
# gpg:                 aka "Gregory Kurz <gregory.kurz@free.fr>" [full]
# gpg:                 aka "[jpeg image of size 3330]" [full]
# Primary key fingerprint: B482 8BAF 9431 40CE F2A3  4910 71D4 D5E5 822F 73D6

* remotes/gkurz/tags/9p-next-2020-05-26:
  xen/9pfs: increase max ring order to 9
  xen/9pfs: yield when there isn't enough room on the ring
  Revert "9p: init_in_iov_from_pdu can truncate the size"
  9p: Lock directory streams with a CoMutex
  9pfs: include linux/limits.h for XATTR_SIZE_MAX

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