KVM: arm64: gic-v5: Handle userspace accesses to IRS MMIO region

As part of saving and restoring the state of a GICv5-based system,
userspace must save and restore the IRS MMIO registers. These include
important information such as the guest IST configuration, and KVM
must present consistent state to the guest after migration.

Introduce KVM_DEV_ARM_VGIC_GRP_IRS_REGS and provide accessors to read
and write the virtual IRS register state. This is modelled on the
GICv3 ITS register interface, as the migration requirements are
broadly the same.

Reuse the guest MMIO handlers where userspace and guest accesses have
the same semantics. Add userspace-specific handling where restoring a
register image must not trigger the operation associated with a guest
MMIO write.

Validate restored ID register fields against the capabilities KVM and
the host can support. Restore the emulated IST configuration without
allocating or freeing a host IST, and report operation status
registers as idle. Accept writes to IRS_SPI_CFGR, IRS_IIDR, and
IRS_AIDR without changing their state, allowing userspace to replay
the values it previously read.

Restoring IRS_IST_BASER.Valid recreates the guest-visible
configuration, but the host LPI IST cannot be allocated until
userspace supplies its contents. Track this as a pending LPI IST
restore and reject guest entry until userspace restores the IST
through KVM_DEV_ARM_VGIC_GRP_IST.

Signed-off-by: Sascha Bischoff <sascha.bischoff@arm.com>
Link: https://patch.msgid.link/20260904113404.4051341-33-sascha.bischoff@arm.com
Signed-off-by: Marc Zyngier <maz@kernel.org>

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