Implement facilities for handling unexpected physical removal of block devices

[nvesely]

Referencing #1072, we should keep an eye out for adding this functionality, if Xen in the future develops monitoring facilities in libxl:

Currently (R3.0) device must be detached from VM before physically unplugging it. Xen toolstack used in R3.0 (libxl + libvirt) does not have any device monitoring mechanisms. If the device is physically removed, the toolstack will still think the device is present and connected (also some settings of xen block frontend and backend will not be cleaned up because of that). The only way to tell the toolstack that device is no longer connected, is to detach it from VM (action initiated by the user, through qubes manager or qvm-block, not by backend driver in response to disconnecting the device). But if the device is no longer there, such detach action would fail.

There is a way to manually clean that mess (basically reconnect the device using low level commands, and then properly detach it), but it isn't anything we want to use in any way. In R2 we had a nasty hack for that, because toolstack in 4.1 was even more primitive in this manner (it hadn't track on connected devices). But it isn't applicable to R3. The proper fix would be to implement device monitoring facilities in libxl, then use that API in libvirt. But this a lot work and probably won't happen anytime soon...

-- @marmarek