qemu-ga: fixes for win32 build of qemu-ga

Various stubs and #ifdefs to compile for Windows using mingw
cross-build. Still has 1 linker error due to a dependency on the
forthcoming win32 versions of the GAChannel/transport class.
diff --git a/configure b/configure
index 037f7f7..a1f4a6b 100755
--- a/configure
+++ b/configure
@@ -509,7 +509,7 @@
   bindir="\${prefix}"
   sysconfdir="\${prefix}"
   confsuffix=""
-  guest_agent="no"
+  libs_qga="-lws2_32 -lwinmm $lib_qga"
 fi
 
 werror=""