vmstate: Add a VSTRUCT type

The VMS_STRUCT has no way to specify which version of a structure
to use.  Add a type and a new field to allow the specific version
of a structure to be used.

Signed-off-by: Corey Minyard <cminyard@mvista.com>
Message-Id: <1524670052-28373-2-git-send-email-minyard@acm.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>