bootindex: add a setter/getter functions wrapper for bootindex property

bootindex: add a setter/getter functions wrapper for bootindex property

when we remove bootindex form qdev.property to qom.property,
we can use those functions set/get bootindex property for all
correlative devices. Meanwhile set the initial value of
bootindex to -1.

Signed-off-by: Gonglei <arei.gonglei@huawei.com>
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>