env_process: Fix regression with non qemu test types

All the fixes required for auto_cpu_model ended up
requiring that we interact with qemu directly on
pre/postprocessing code. However, there was no
restriction put on which vm_type the auto config
should happen.

Therefore, restrict the auto_cpu_model feature to
qemu vms and fix the problem.

CC: Chris Evich <cevich@redhat.com>
CC: Yu Mingfei <yumingfei@cn.fujitsu.com>
Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>