build: Fix linking failure for qemu-ga

build: Fix linking failure for qemu-ga

This patch will fix the following linking failed:

  LINK  qemu-ga
gcc: error: qga/../qapi-generated/qga-qapi-types.o: No such file or directory
gcc: error: qga/../qapi-generated/qga-qapi-visit.o: No such file or directory
gcc: error: qga/../qapi-generated/qga-qmp-marshal.o: No such file or directory
make: *** [qemu-ga] Error 1

Commit cdc976b040f2760103ce2fb49f4d504093a7163f changes the
dependencies of qemu-ga to depend "../qapi-generated/qga-qapi-types.o",
which will be expanded to "qga/../qapi-generated/qga-qapi-types.o" when
building qemu-ga.

In top-level Makefile, we defined a target "qapi-generated/qga-qapi-types.o"
which was not equal to "qga/../qapi-generated/qga-qapi-types" in the
Makefile world. So "No such file" error happened when qemu-ga was linking.

The easy approach to fix is to change the target name to
"qga/../qapi-generated/qga-qapi-types.o", but it is weird.

So, in order to solve it more graciously, I move those temporary
files(qga-qapi-*.{c,h}) qemu-ga depends on to qemu-ga/qapi-generated,
this makes dependencies more clearer.

Signed-off-by: Dunrong Huang <riegamaths@gmail.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>