migration/multifd: Move nocomp code into multifd-nocomp.c

migration/multifd: Move nocomp code into multifd-nocomp.c

In preparation for adding new payload types to multifd, move most of
the no-compression code into multifd-nocomp.c. Let's try to keep a
semblance of layering by not mixing general multifd control flow with
the details of transmitting pages of ram.

There are still some pieces leftover, namely the p->normal, p->zero,
etc variables that we use for zero page tracking and the packet
allocation which is heavily dependent on the ram code.

Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Fabiano Rosas <farosas@suse.de>