VMD has it's own PCI/MSI interrupt domain which is not in any way depending
on the x86 vector domain. PCI devices behind VMD share the VMD MSIX vector
entries via a VMD specific message translation to the actual VMD MSIX
vector. The VMD device interrupt handler for the VMD MSIX vectors invokes
all interrupt handlers of the devices which share a vector.

Making the x86 vector domain the actual parent of the VMD irq domain is
pointless and actually counterproductive. When a device interrupt is
requested then it will activate the interrupt which traverses down the
hierarchy and consumes an interrupt vector in the vector domain which is
never used.

The domain is self contained and has no parent dependencies, so just hand
in NULL for the parent and be done with it.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20200826112330.928952181@linutronix.de