KVM: selftests: Add VGICv5 IST attribute tests

Add API coverage for the VGICv5 IST attribute group. This does not
test that IST save/restore preserves interrupt state; it only checks
the attribute plumbing and the cases KVM must reject before the
end-to-end restore path can rely on the interface.

Verify that KVM reports the IST attribute group, rejects unknown
attributes, rejects save/restore before initialization, rejects bad
userspace pointers, and rejects IST restore after the VM has run.

Also cover the userspace IST descriptor validation: a VM with SPIs
must provide a SPI IST buffer of the expected size, and LPI storage
must not be supplied when no LPI IST is configured.

Signed-off-by: Sascha Bischoff <sascha.bischoff@arm.com>
Link: https://patch.msgid.link/20260904113404.4051341-44-sascha.bischoff@arm.com
Signed-off-by: Marc Zyngier <maz@kernel.org>

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