hostmem-file: plug a small leak

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <1460566660-19241-1-git-send-email-marcandre.lureau@redhat.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
