nodedev: handle mdevs that disappear from mdevctl

nodedev: handle mdevs that disappear from mdevctl

mdevctl does not currently provide any events when the list of defined
devices changes, so we will need to poll mdevctl for the list of defined
devices periodically. When a mediated device no longer exists from one
iteration to the next, we need to treat it as an "undefine" event.

When we get such an event, we remove the device from the list if it's
not active. Otherwise, we simply mark it as non-persistent.

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