KVM: arm64: selftests: Check a feature hidden in an ID register is UNDEF

Userspace can hide a feature from a guest by clearing its field in a
writable ID register, and KVM then makes the feature's i

KVM: arm64: selftests: Check a feature hidden in an ID register is UNDEF

Userspace can hide a feature from a guest by clearing its field in a
writable ID register, and KVM then makes the feature's instructions
UNDEFINED in the guest by trapping or disabling them. No selftest checks
that.

Add a test that runs the instruction of each of TLBI OS, MOPS, TCR2_EL1
and FPMR once with its field as advertised and once with it cleared, and
expects an UNDEF only when cleared. A feature the vCPU doesn't advertise
is skipped, as is hidden TLBI OS on a CPU with neither FGT nor
FEAT_EVT2, where KVM can't trap it.

Signed-off-by: Fuad Tabba <fuad.tabba@linux.dev>
Link: https://patch.msgid.link/20260929090031.3829185-5-fuad.tabba@linux.dev
Signed-off-by: Oliver Upton <oupton@kernel.org>

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