package/qemu: add host-python3 as an explicit dependency

qemu requires python in its configre script. Yet host-python was
not listed as one of the package's dependencies. If no other package
requested host-python, then configuring this package will fail since
it won't find any executable named python in the host dir.

In order to reproduce this issue you must not have python2 installed
on your host machine.

Signed-off-by: Avi Shukron <avraham.shukron@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>