qemu: Clean up qemuDomainDefaultUSBControllerModel()

qemu: Clean up qemuDomainDefaultUSBControllerModel()

Switch from the current approach, in which an initial and
likely poor default is picked and then a better one later
overwrites it, to the more common and easy to reason about
pattern where the value is returned directly as soon as
possible.

To make things easier to understand and more maintainable,
the various architectures for which we have explicit
handling are each taken care of separately, with no falling
through to the default behavior.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>