kvm: fix incorrect length in a loop over kvm dirty pages map

QEMU allocates a map enough for 4k pages. However the system page size
can be 64K (for example on POWER) and the host kernel uses only a small
part of it as one big stores a dirty flag for 16 pages 4K each,
the hpratio variable stores this ratio and
the kvm_get_dirty_pages_log_range function handles it correctly.

However kvm_get_dirty_pages_log_range still goes beyond the data
provided by the host kernel which is not correct. It does not cause
errors at the moment as the whole bitmap is zeroed before doing KVM ioctl.

The patch reduces number of iterations over the map.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Alexander Graf <agraf@suse.de>
