Merge remote-tracking branch 'remotes/mdroth/tags/qga-pull-2016-04-19-tag' into staging

qemu-ga patch queue for 2.6

* fixes inadvertant change that unconditionally disables qemu-ga unit test
* fixes make check failures when building with --disable-guest-agent that
  were present visible before the unit test was inadvertantly disabled.

# gpg: Signature made Tue 19 Apr 2016 23:30:09 BST using RSA key ID F108B584
# gpg: Good signature from "Michael Roth <flukshun@gmail.com>"
# gpg:                 aka "Michael Roth <mdroth@utexas.edu>"
# gpg:                 aka "Michael Roth <mdroth@linux.vnet.ibm.com>"

* remotes/mdroth/tags/qga-pull-2016-04-19-tag:
  qemu-ga: do not run qga test when guest agent disabled

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