Leave ppt devices in the host domain when they are not attached to a VM.

[jhb]

Summary

Leave ppt devices in the host domain when they are not attached to a VM.

This allows a pass through device to be reset to a normal device driver
on the host and reused on the host. ppt devices are now always active in
some I/O MMU domain when the I/O MMU is active, either the host domain
or the domain of a VM they are attached to.

Test Plan

• start a VM using device pass through and verify via dmardump that the device moves back to domain 1 after the VM shuts down. (Previously it just disappeared from the dump since the entry was disabled.)

[jhb]

FYI, I have moved this out to the ppt driver due to other changes I have in flight. Will update once I have tested the new version, but you can ignore this for now until I upload the next version.

[jhb]

• Place all PCI devices in the host domain during initialization.
• Change ppt to move devices into the host domain instead of removing them.
• Remove device before adding it to the host domain.
• Remove device from host domain before adding it to the VM domain.

[jhb]

Another approach perhaps would be to fix iommu_add_device() to permit "moving" a device between domains. I think this mostly just means removing the explicit panic from vtd_add_device() (though perhaps there are other gotchas there I am not aware of?). If we go that route, I would remove the calls to iommu_remove_device() from ppt.c.

[jhb]

Rebase.