Merge remote-tracking branch 'remotes/juanquintela/tags/migration-20211106-pull-reque...

Migration Pull request

- fix vhost-user crash when using postcopy (me)
- fix incorrect tag for docs (hyman)

Please apply, Juan.

# gpg: Signature made Sat 06 Nov 2021 07:29:37 PM EDT
# gpg:                using RSA key 1899FF8EDEBF58CCEE034B82F487EF185872D723
# gpg: Good signature from "Juan Quintela <[email protected]>" [full]
# gpg:                 aka "Juan Quintela <[email protected]>" [full]

* remotes/juanquintela/tags/migration-20211106-pull-request:
  docs: fix qemu incorrect tag
  migration: Check that postcopy fd's are not NULL

Signed-off-by: Richard Henderson <[email protected]>