KVM: selftests: Move kvm_create_max_vcpus test to generic code

There is nothing x86-specific in the test apart from the VM_MODE_P52V48_4K
which we can now replace with VM_MODE_DEFAULT. Thus let's move the file to
the main folder and enable it for aarch64 and s390x, too.

Reviewed-by: Andrew Jones <drjones@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20190523164309.13345-10-thuth@redhat.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>