Right now when building the qemu command line, we try to do various
unconditional validations of the guest CPU against the host CPU. However
this checks are overly applied. The only time we should use the checks
are:

- The user requests host-model/host-passthrough, or

- When KVM is requsted. CPU features requested in TCG mode are always
  emulated by qemu and are independent of the host CPU, so no host CPU
  checks should be performed.

Right now if trying to specify a CPU for arm on an x86 host, it attempts
to do non-sensical validation and falls over.

Switch all the test cases that were intending to test CPU validation to
use KVM, so they continue to test the intended code.

Amend some aarch64 XML tests with a CPU model, to ensure things work
correctly.

This patch changes network device type used by default from rtl8139
to virtio when architecture type is aarch64 and machine type is virt.
Qemu doesn't support any other machine types for aarch64 right now and
we can't make any other aarch64-specific tuning in this function yet.
Signed-off-by: NOleg Strikov <oleg.strikov@canonical.com>