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/Makefile b/Makefile
index c67493e..ad1e627 100644
--- a/Makefile
+++ b/Makefile
@@ -202,7 +202,7 @@
 $(QGALIB_OBJ): $(QGALIB_GEN) $(GENERATED_HEADERS)
 $(qga-obj-y) qemu-ga.o: $(QGALIB_GEN) $(GENERATED_HEADERS)
 
-qemu-ga$(EXESUF): qemu-ga.o $(qga-obj-y) $(qapi-obj-y) $(tools-obj-y) $(qobject-obj-y) $(version-obj-y) $(QGALIB_OBJ)
+qemu-ga$(EXESUF): qemu-ga.o $(qga-obj-y) $(tools-obj-y) $(qapi-obj-y) $(qobject-obj-y) $(version-obj-y) $(QGALIB_OBJ)
 
 QEMULIBS=libhw32 libhw64 libuser libdis libdis-user