virttest.bootstrap: Don't change a list when transversing it

Currently, when creating config files, autotest tries to find out
the config files that need to be overridden in testdir/cfg/*.cfg.

But it tries to pop the element for the list during traversing
itself, this is very dangerous, and it might cause an error when
running get_started.py, because the list wasn't updated yet, but
the transverse index wasn't updated.

Signed-off-by: Mike Qiu <qiudayu@linux.vnet.ibm.com>
virttest.qemu_installer: Autodetect qemu based on host arch

When detecting appropriate qemu names by default to run the
test suite, right now x86_64 is assumed. So, in order to
support other host architectures, such as ppc64, guess the
appropriate qemu name based on the host architecture.

Signed-off-by: Mike Qiu <qiudayu@linux.vnet.ibm.com>