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

Fix global property and -cpu handling bug

This bug fix was supposed to be applied just after 2.8.0 was
released, but it slipped through the cracks. Sending it now for
the next -rc.

# gpg: Signature made Tue 14 Mar 2017 20:04:50 GMT
# gpg:                using RSA key 0x2807936F984DC5A6
# gpg: Good signature from "Eduardo Habkost <ehabkost@redhat.com>"
# Primary key fingerprint: 5A32 2FD5 ABC4 D3DB ACCF  D1AA 2807 936F 984D C5A6

* remotes/ehabkost/tags/machine-pull-request:
  machine: Convert abstract typename on compat_props to subclass names

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