tests: extract KVM configuration into framework/kvm.py

Move the body of Microvm.basic_config into a standalone
kvm_basic_config helper, alongside the has_kvm probe. Microvm still
carries its inline copy until the vm_backend dimension lands and
rewires it to delegate.

This prepares for the vm_backend dimension, so its commit contains only
new logic rather than relocated KVM code.

Signed-off-by: James Curtis <jxcurtis@amazon.co.uk>