Add 'devctl delete' that calls device_delete_child().

[jhb]

Summary

Add 'devctl delete' that calls device_delete_child().

'devctl delete' can be used to delete a device that is no longer present.
As a foot-shooting measure, 'delete' will not delete a device unless it's
parent bus says it is no longer present. This can be overridden by passing
the force ('-f') flag.

Note that this command should be used with care. If a device is deleted
that is actually present it can't be resurrected unless the parent bus
driver supports rescans.

Test Plan

• delete a device on a PCI bus and then use devctl rescan to bring it back