Can't define a cpu_model from a different architecture

[Belmiro Moreira]

"""
It would be great if Nova supports instances with a different architecture than the host.
My use case is to be run aarch64 guests in a x86_64 compute node.
"""

In order to create an aarch64 guest in an x86_64 compute node we need to define the emulated CPU.
However, Nova doesn't allow to define a CPU model that doesn't match with the host architecture.

For example:
CONF.libvirt.virt_type=qemu
CONF.libvirt.cpu_model=cortex-a57
CONF.libvirt.cpu_mode=custom

It fails with:
nova.exception.InvalidCPUInfo: Configured CPU model: cortex-a57 is not correct, or your host CPU arch does not support this model. Please correct your config and try again.

The problem is related with the this nova check in driver.py:
if cpu_info['arch'] not in (fields.Architecture.I686,
                            fields.Architecture.X86_64,
                            fields.Architecture.PPC64,
                            fields.Architecture.PPC64LE,
                            fields.Architecture.PPC):
    return model

Again, it's relying the host architecture for the x86_64.

Environment
===========

Tested using the master branch (29/10/2020)

Other
=====

I'm now opening target bugs for the generic issue reported in https://bugs.launchpad.net/nova/+bug/1863728