migration: Rename duplicate to zero_pages

Rest of counters that refer to pages has a _pages suffix.
And historically, this showed the number of pages composed of the same
character, here comes the name "duplicated".  But since years ago, it
refers to the number of zero_pages.

Signed-off-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
3 files changed