Makefile: add qga-vss-dll-obj-y to nested variables

The build rule for qga/vss-win32/qga-vss.dll is broken by commit
ba1183da9a10b94611cad88c44a5c6df005f9b55, because it misses
qga-vss-dll-obj-y in the list of nested variables.
This fixes build of qga-vss.dll by adding qga-vss-dll-obj-y to the list.

Signed-off-by: Tomoki Sekiyama <tomoki.sekiyama@hds.com>
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
vss-win32: Fix build with mingw64-headers-3.1.0

In mingw64-headers-3.1.0, definition of _com_issue_error() is added, which
conflicts with definition in install.cpp. This adds version checking for
mingw headers to disable the definition when the headers>=3.1 is used.

Signed-off-by: Tomoki Sekiyama <tomoki.sekiyama@hds.com>
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
Merge remote-tracking branch 'remotes/mdroth/qga-pull-2014-4-7' into staging

* remotes/mdroth/qga-pull-2014-4-7:
  vss-win32: Fix build with mingw64-headers-3.1.0
  Makefile: add qga-vss-dll-obj-y to nested variables

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