package/qemu: fix host-qemu variable names

With the upcoming introduction of qemu-on-target, we need to properly
separate the variables used for the host qemu, from the variables
used for the target qemu.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Francois Perrad <fperrad@gmail.com>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>