qemu.pl: make clean + checkout + make
qemu.pl: make clean + checkout + make

[ Instead of checkout + make clean + make ]

This way we make clean on (hopefully) the original tree we
built from.

Signed-off-by: Emilio G. Cota <201e881bfa0e9543bba00630d033ebfb953e7c40@braap.org>

cota/dbt-bench