qdev: Check if unplug handler exists before calling it

A bus may have hotplugging enabled but not have the 'unplug'
callback defined, which would lead to a crash on trying to
unplug a device on the bus.

Fix by introducing an assert to check if the callback is valid.

Signed-off-by: Amit Shah <amit.shah@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
