savevm: fix corruption in vmstate_subsection_load().

Although it's rare to happen in live migration, when the head of a
byte stream contains 0x05 which is the marker of subsection, the
loader gets corrupted because vmstate_subsection_load() continues even
the device doesn't require it.  This patch adds a checker whether
subsection is needed, and skips following routines if not needed.

Signed-off-by: Yoshiaki Tamura <tamura.yoshiaki@lab.ntt.co.jp>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
