memory: Fix start offset for bitmap log_clear hook

memory: Fix start offset for bitmap log_clear hook

Currently only the final page offset is being passed to the `log_clear`
hook via `memory_region_clear_dirty_bitmap` after it is used as an
iterator in `cpu_physical_memory_test_and_clear_dirty`. This patch
corrects the start address and size of the region.

Signed-off-by: Matt Borgerson <contact@mborgerson.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>