nodedev: refactor nodeDeviceFindNewDevice()

nodedev: refactor nodeDeviceFindNewDevice()

In preparation for creating mediated devices in libvirt, we will need to
wait for new mediated devices to be created as well. Refactor
nodeDeviceFindNewDevice() so that we can re-use the main logic from this
function to wait for different device types by passing a different
'find' function.

Signed-off-by: Jonathon Jongsma <jjongsma@redhat.com>
Reviewed-by: Erik Skultety <eskultet@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>