build: move qga/ objects to nested Makefile.objs

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
diff --git a/qga/Makefile.objs b/qga/Makefile.objs
new file mode 100644
index 0000000..6a4d843
--- /dev/null
+++ b/qga/Makefile.objs
@@ -0,0 +1,3 @@
+qga-obj-y = commands.o guest-agent-command-state.o
+qga-obj-$(CONFIG_POSIX) += commands-posix.o channel-posix.o
+qga-obj-$(CONFIG_WIN32) += commands-win32.o channel-win32.o service-win32.o