KVM: arm64: GICv3: Force exit to sync ICH_HCR_EL2.En

FEAT_NV2 is pretty terrible for anything that tries to enforce immediate
effects, and writing to ICH_HCR_EL2 in the hope to disable a maintenance
interrupt is vain. This only hits memory, and the guest hasn't cleared
anything -- the MI will fire.

For example, running the vgic_irq test under NV results in about 800
maintenance interrupts being actually handled by the L1 guest,
when none were expected.

As a cheap workaround, read back ICH_MISR_EL2 after writing 0 to
ICH_HCR_EL2. This is very cheap on real HW, and causes a trap to
the host in NV, giving it the opportunity to retire the pending MI.
With this, the above test runs to completion without any MI being
actually handled.

Yes, this is really poor...

Tested-by: Fuad Tabba <tabba@google.com>
Reviewed-by: Fuad Tabba <tabba@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Tested-by: Mark Brown <broonie@kernel.org>
Link: https://msgid.link/20251120172540.2267180-37-maz@kernel.org
Signed-off-by: Oliver Upton <oupton@kernel.org>

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