qemu-nbd: move client handling to nbd.c

qemu-nbd: move client handling to nbd.c

This patch sets up the fd handler in nbd.c instead of qemu-nbd.c.  It
introduces NBDClient, which wraps the arguments to nbd_trip in a single
structure, so that we can add a notifier to it.  This way, qemu-nbd can
know about disconnections.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>