qemu: Default to no USB and no memballoon for new architectures

qemu: Default to no USB and no memballoon for new architectures

The current defaults, that can be altered on a per-architecture
basis, are derived from the historical x86 behavior.

Every time support for a new architecture is added to libvirt,
care must be taken to override these default: if that doesn't
happen, guests will end up with additional hardware, which is
something that's generally undesirable.

Turn things around, and require architectures to explicitly
ask for the devices to be created by default instead. The
behavior for existing architectures is preserved.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>