KVM: selftests: Remove duplicate guest mode handling

Remove the duplication code in run_test() of dirty_log_test because
after some reordering of functions now we can directly use the outcome
of vm_create().

Meanwhile, with the new VM_MODE_PXXV48_4K, we can safely revert
b442324b too where we stick the x86_64 PA width to 39 bits for
dirty_log_test.

Reviewed-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>