New API: inspect-get-windows-current-control-set

This returns the actual registry key corresponding to
CurrentControlSet (eg. it might be "ControlSet001").

Previously the inspection code was hard-coding ControlSet001.  Now we
use the correct control set, and also make it available to callers
through the API.

This commit also updates the virt-dhcp-address example so it uses this
new API.

virt-inspector displays the current control set when available.