package/qemu: use autotools-package infrastructure

Turns out that, with a little bit of tweaking, we can use
the autotools-package infrastructure to build QEMU.

That's better than defining all the _CMDS and using the
generic-package infra.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Francois Perrad <fperrad@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>