hostmem-file: plug a small leak

# gpg: Signature made Fri 15 Apr 2016 17:30:42 BST using RSA key ID EB918653
# gpg: Good signature from "Markus Armbruster <armbru@redhat.com>"
# gpg:                 aka "Markus Armbruster <armbru@pond.sub.org>"

* remotes/armbru/tags/pull-backends-2016-04-15:
  hostmem-file: plug a small leak

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