qemu_hotplug: Rework media changing process

qemu_hotplug: Rework media changing process

https://bugzilla.redhat.com/show_bug.cgi?id=892289

It seems like with new udev within guest OS, the tray is locked,
so we need to:
- 'eject'
- wait for tray to open
- 'change'

Moreover, even when doing bare 'eject', we should check for
'tray_open' as guest may have locked the tray. However, the
waiting phase shouldn't be unbounded, so I've chosen 10 retries
maximum, each per 500ms. This should give enough time for guest
to eject a media and open the tray.