nbd: Split nbd.c

We have NBD server code and client code, all mixed in a file. Now split
them into separate files under nbd/, and update MAINTAINERS.

filter_nbd for iotest 083 is updated to keep the log filtered out.

Signed-off-by: Fam Zheng <famz@redhat.com>
Message-Id: <1452760863-25350-3-git-send-email-famz@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
diff --git a/nbd/Makefile.objs b/nbd/Makefile.objs
new file mode 100644
index 0000000..eb3dd44
--- /dev/null
+++ b/nbd/Makefile.objs
@@ -0,0 +1 @@
+block-obj-y += server.o client.o common.o