qemuwrapper: Explicitly exit in case of no qemu supported for target

qemuwrapper: Explicitly exit in case of no qemu supported for target

Running qemu for userspace code on unsupported target binaries might
be bad idea because qemu could say running in endless loop instead
of crashing due to illegal instruction or unsupported binary format.

While this is qemu bug we should avoid hitting it by explicitly exiting
from the wrapper when qemu backfill considered for machine.

Behaviour was observed in do_rootfs stage when building on IBM Power 8
host for PowerPC e7400 target.

Signed-off-by: Serhey Popovych <serhe.popovych@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>