This refactoring is necessary to support hotplug detach of
type=hostdev network devices, but needs to be in a separate patch to
make potential debugging of regressions more practical.

Rather than the lowest level functions searching for a matching
device, the search is now done in the toplevel function, and an
intermediate-level function (qemuDomainDetachThisHostDevice()), which
expects that the device's entry is already found, is called (this
intermediate function will be called by qemuDomainDetachNetDevice() in
order to support detach of type=hostdev net devices)

This patch should result in 0 differences in functionality.