KVM: selftests: Add arch specific post vm creation hook

Add arch specific API kvm_arch_vm_post_create to perform any required setup
after VM creation.

Suggested-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Andrew Jones <andrew.jones@linux.dev>
Reviewed-by: Peter Gonda <pgonda@google.com>
Signed-off-by: Vishal Annapurve <vannapurve@google.com>
Link: https://lore.kernel.org/r/20221115213845.3348210-4-vannapurve@google.com


[sean: place x86's implementation by vm_arch_vcpu_add()]
Signed-off-by: Sean Christopherson <seanjc@google.com>