Merge remote-tracking branch 'remotes/ehabkost/tags/machine-pull-request' into staging

cpu: crash fix (don't allow negative core id)

# gpg: Signature made Thu 03 Aug 2017 18:57:41 BST
# gpg:                using RSA key 0x2807936F984DC5A6
# gpg: Good signature from "Eduardo Habkost <[email protected]>"
# Primary key fingerprint: 5A32 2FD5 ABC4 D3DB ACCF  D1AA 2807 936F 984D C5A6

* remotes/ehabkost/tags/machine-pull-request:
  cpu: don't allow negative core id

Signed-off-by: Peter Maydell <[email protected]>