[CFT] patch for suspend on systems with shared interrupts

I am working on a patch to improve stability of suspend (and resume) on systems
where an interrupt line (legacy PCI) is shared by multiple devices.

If you have such a system and you experience hangs during suspend or resume,
could you please test the patch?
It's easy to check if you have shared interrupt.  For example, you can run
devinfo -u and check 'Interrupt request lines' of the output for any repeating
numbers.

I would also appreciate any reviews of the change.  Especially, because it
introduces new methods to the bus interface.  I would like to ensure that the
new methods are reasonable.

The change is here: https://reviews.freebsd.org/D15755

Thank you.
-- 
Andriy Gapon