migration: Correctly handle subsections with no 'needed' function

migration: Correctly handle subsections with no 'needed' function

Currently the vmstate subsection handling code treats a subsection
with no 'needed' function pointer as if it were the subsection
list terminator, so the subsection is never transferred and nor
is any subsection following it in the list.

Handle NULL 'needed' function pointers in subsections in the same
way that we do for top level VMStateDescription structures:
treat the subsection as always being needed.

This doesn't change behaviour for the current set of devices
in the tree, because all subsections declare a 'needed' function.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>