qapi/qom: Add ObjectOptions for dbus-vmstate

This adds a QAPI schema for the properties of the dbus-vmstate object.

A list represented as a comma separated string is clearly not very
QAPI-like, but for now just describe the existing interface.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Acked-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>