uffd.c: move the code out of the 'main' function

Most of the UFFD logic was in the function uffd_listen() which was
directly called from crtools.c. In preparation for the remote lazy
restore most of the code has been moved to separate function for better
integration of the network functionality.

Signed-off-by: Adrian Reber <areber@redhat.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>