[PATCH] Fix bug for vcpu hotplug

[Liu, Jinsong]

Recently seabios implement vcpu hotplug infrastructure.
During test, we found qemu-kvm has a bug result in guestos shutdown when vcpu hotadd.
This patch is to fix the bug, mark bus->allow_hotplug as 1 after qdev_hotplug init done.

Signed-off-by: Liu, Jinsong