KVM: selftests: Introduce num-pages conversion utilities

Guests and hosts don't have to have the same page size. This means
calculations are necessary when selecting the number of guest pages
to allocate in order to ensure the number is compatible with the
host. Provide utilities to help with those calculations and apply
them where appropriate.

We also revert commit bffed38d ("kvm: selftests: aarch64:
dirty_log_test: fix unaligned memslot size") and then use
vm_adjust_num_guest_pages() there instead.

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