migration: Split registration functions from vmstate.h

They are indpendent, and nowadays almost every device register things
with qdev->vmsd.

Signed-off-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Peter Xu <peterx@redhat.com>
diff --git a/slirp/slirp.c b/slirp/slirp.c
index 2386493..1d67568 100644
--- a/slirp/slirp.c
+++ b/slirp/slirp.c
@@ -26,6 +26,7 @@
 #include "qemu/timer.h"
 #include "qemu/error-report.h"
 #include "chardev/char-fe.h"
+#include "migration/register.h"
 #include "slirp.h"
 #include "hw/hw.h"
 #include "qemu/cutils.h"