KVM: arm64: Add KVM_ARM_VCPU_PMU_V3_STRICT vCPU feature

Introduce a new vCPU feature KVM_ARM_VCPU_PMU_V3_STRICT. When set, KVM
does not create a default PMU when initializing the vCPU, and userspace
must select one explicitly via KVM_ARM_VCPU_PMU_V3_SET_PMU before the
first KVM_RUN.

The flag forces the VMM to be aware of the PMU implementation of the
guest to be created, so that certain information about the PMU becomes
deterministic (if on a heterogeneous system) and becomes safe to be
exposed to the guest. It can be used as an umbrella flag to gate future
PMUv3 UAPI changes.

When no default PMU is created, kvm->arch.arm_pmu stays NULL until SET_PMU
runs, so kvm_arm_pmu_v3_init() now refuses to run if kvm->arch.arm_pmu
is NULL.

Signed-off-by: Congkai Tan <congkai@amazon.com>
Reviewed-by: Geoff Blake <blakgeof@amazon.com>
Reviewed-by: Haris Okanovic <harisokn@amazon.com>
Reviewed-by: Stanislav Spassov <stanspas@amazon.de>
Co-developed-by: Oliver Upton <oupton@kernel.org>
Reviewed-by: Fuad Tabba <fuad.tabba@linux.dev>
Tested-by: Fuad Tabba <fuad.tabba@linux.dev>
Link: https://patch.msgid.link/20260722202702.4165917-5-congkai@amazon.com
Signed-off-by: Oliver Upton <oupton@kernel.org>

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