migration: Create x-multifd-page-count parameter

Indicates how many pages we are going to send in each batch to a multifd
thread.

Signed-off-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>

--

Be consistent with defaults and documentation
Use new DEFINE_PROP_*
Rename x-multifd-group to x-multifd-page-count