kvm: selftests: dirty_log_test: don't identity map the test mem

It isn't necessary and can even cause problems when testing high
guest physical addresses. This patch leaves the test memory id-
mapped by default, but when using '-t' the test memory virtual
addresses stay the same even though the physical addresses switch
to the topmost valid addresses.

Signed-off-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>