kvm: accept non-mapped memory in kvm_dirty_pages_log_change

It is okay if memory is not mapped into the guest but has dirty logging
enabled.  When this happens, KVM will not do anything and only accesses
from the host will be logged.

This can be triggered by iofuzz.

Reviewed-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>