qemu-ga: Add configure options to specify path to Windows/VSS SDK

To enable VSS support in qemu-ga for Windows, header files included in
VSS SDK are required.
The VSS support is enabled by the configure option like below:
  ./configure --with-vss-sdk="/path/to/VSS SDK"

If the path is omitted, it tries to search the headers from default paths
and VSS support is enabled only if the SDK is found.
VSS support is disabled if --without-vss-sdk or --with-vss-sdk=no is
specified.

VSS SDK is available from:
  http://www.microsoft.com/en-us/download/details.aspx?id=23490



To cross-compile using mingw, you need to setup the SDK on Windows
environments to extract headers. You can also extract the SDK headers on
POSIX environments using scripts/extract-vss-headers and msitools.

In addition, --with-win-sdk="/path/to/Windows SDK" option is also added to
specify path to Windows SDK, which may be used for native-compile of .tlb
file of qemu-ga VSS provider. However, this is usually unnecessary because
pre-compiled .tlb file is included.

Signed-off-by: Tomoki Sekiyama <tomoki.sekiyama@hds.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Michael Roth <mdroth@linux.vnet.ibm.com>
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>