After all the previous patches, spliting the bitmap gets direct.

Note: For some reason, I have to move DIRTY_MEMORY_* definitions to
the beginning of memory.h to make compilation work.
Signed-off-by: NJuan Quintela <quintela@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Reviewed-by: NOrit Wasserman <owasserm@redhat.com>