Merge remote-tracking branch 'remotes/elmarco/tags/slirp-pull-request' into staging

# gpg: Signature made Sun 22 Mar 2020 17:07:31 GMT
# gpg:                using RSA key 87A9BD933F87C606D276F62DDAE8E10975969CE5
# gpg:                issuer "marcandre.lureau@redhat.com"
# gpg: Good signature from "Marc-André Lureau <marcandre.lureau@redhat.com>" [full]
# gpg:                 aka "Marc-André Lureau <marcandre.lureau@gmail.com>" [full]
# Primary key fingerprint: 87A9 BD93 3F87 C606 D276  F62D DAE8 E109 7596 9CE5

* remotes/elmarco/tags/slirp-pull-request:
  slirp: update submodule to v4.2.0+

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