Add the possibility to choose which architecture use to build the wanted
image (--arch). Since this implies that running commands on the guest is
usually not possible when the architecture is different than the host
one, virt-builder will nevertheless try to check whether the host is
compatible with the guest, allowing to run commands only in that case.
The caching scheme is adapted to account for the architecture (with
--print-cache showing the architecture as well).
b1cf6246f3