KVM: selftests: Add VGICv5 IST save/restore coverage

Add end-to-end VGICv5 IST save/restore coverage. The earlier IST
attribute test checks only API rejection cases; this test verifies
that interrupt state saved from the ISTs can be restored into a fresh
VM before first run and then consumed by the guest.

Build a source VM that configures SPI and LPI enable, priority, and
affinity state from the guest. Inject selected SPIs from userspace,
save the IRS register state, and save both the SPI and LPI IST images
through KVM_DEV_ARM_VGIC_GRP_IST using userspace-provided buffers.

Create a restore VM and verify that IST restore is rejected before the
IRS state has been restored. Restore the IRS registers, validate bad
or missing LPI storage, and restore the saved SPI and LPI IST images.

Before the host LPI IST is allocated, verify that userspace can change
or invalidate IRS_IST_BASER. Once the IST images have been restored
and the host LPI IST exists, verify that matching IRS_IST_BASER and
IRS_IST_CFGR values can be replayed, while changing either register or
clearing IRS_IST_BASER.Valid is rejected.

Before running any destination vCPU, save the ISTs again and compare
the re-saved buffers against the original saved buffers so that
dropped serialised state fails independently of interrupt delivery.

Finally, run the restored guest and check pending and enabled SPI and
LPI state across two VPEs, priority ordering through ICC_PCR_EL1,
delivery of an interrupt that was pending but disabled at save time,
and affinity for interrupts injected after restore.

Signed-off-by: Sascha Bischoff <sascha.bischoff@arm.com>
Link: https://patch.msgid.link/20260904113404.4051341-49-sascha.bischoff@arm.com
Signed-off-by: Marc Zyngier <maz@kernel.org>

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