KVM: arm64: Document ordering requirements for irqbypass

One of the not-so-obvious requirements for restoring a VM is ensuring
that the vITS has been restored _before_ creating any irqbypass mappings.
This is because KVM needs to get the guest translation for MSIs to
correctly assemble the vLPI mapping getting installed in the physical
ITS.

Document the restore ordering requirements necessary for GICv4 vLPI
injection to work.

Acked-by: Marc Zyngier <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Oliver Upton <[e-mail]>

Documentation/virt/kvm/devices/arm-vgic-its.rst

···
 ITS Restore Sequence:
 ---------------------
 
-The following ordering must be followed when restoring the GIC and the ITS:
+The following ordering must be followed when restoring the GIC, ITS, and
+KVM_IRQFD assignments:
 
 a) restore all guest memory and create vcpus
 b) restore all redistributors
···
      2. Restore all other ``GITS_`` registers, except GITS_CTLR!
      3. Load the ITS table data (KVM_DEV_ARM_ITS_RESTORE_TABLES)
      4. Restore GITS_CTLR
+
+e) restore KVM_IRQFD assignments for MSIs
 
 Then vcpus can be started.