Documentation: KVM: Add the VGICv5 IRS save/restore sequences

Document the ordering requirements for saving and restoring the GICv5
IRS state.

The IST state is exposed through KVM_DEV_ARM_VGIC_GRP_IST and is
copied to or from userspace-provided buffers. Saving the ISTs
therefore does not update guest memory, so userspace does not need to
serialize guest memory after saving the IST state.

On restore, userspace must create the vCPUs, configure the IRS
address, restore the number of SPIs, and initialize the VGIC before
restoring IRS state. The IRS MMIO registers may be restored in any
order, but the final IRS_IDR* state and the IRS_IST_CFGR/IRS_IST_BASER
state that describes any LPI IST must be restored before
KVM_DEV_ARM_VGIC_GRP_IST.  KVM uses that state to validate the
userspace IST buffers and allocate any host LPI IST.

The IRS_*_STATUSR registers are observational state in KVM. Userspace
may save them for validation or debugging, but they are not required
as restore input and do not need to be replayed.

Signed-off-by: Sascha Bischoff <sascha.bischoff@arm.com>
Link: https://patch.msgid.link/20260904113404.4051341-40-sascha.bischoff@arm.com
Signed-off-by: Marc Zyngier <maz@kernel.org>

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