HMP fixes for 2.12

# gpg: Signature made Tue 20 Mar 2018 12:39:24 GMT
# gpg:                using RSA key 0516331EBC5BFDE7
# gpg: Good signature from "Dr. David Alan Gilbert (RH2) <dgilbert@redhat.com>"
# Primary key fingerprint: 45F5 C71B 4A0C B7FB 977A  9FA9 0516 331E BC5B FDE7

* remotes/dgilbert/tags/pull-hmp-20180320:
  hmp: free sev info
  HMP: Initialize err before using

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