[SRU bionic] LP#1792099 -- fix vfio device removal deadlocks

[Andy Whitcroft]

A customer reports that when performing device hotplug on devices under
vfio.  Essentially the kernel is holding a device lock over a userspace
interaction.  This userspace interaction may need the same device lock,
leading two threads to deadlock.  Drop this lock over the interaction
with userspace and revalidate.

Following this email is a patch for bionic which implements this.
Proposing for SRU to bionic.

-apw