KVM: arm64: Pass PSCI calls to userspace

When the KVM_CAP_ARM_PSCI_TO_USER capability is available, userspace can
request to handle PSCI calls.

This is required for virtual CPU hotplug to allow the VMM to enforce the
online/offline policy it has advertised via ACPI. By managing PSCI in
user-space, the VMM is able to return PSCI_DENIED when the guest attempts
to bring a disabled vCPU online.
Without this, the VMM is only able to not-run the vCPU, the kernel will
have already returned PSCI_SUCCESS to the guest. This results in
timeouts during boot as the OS must wait for the secondary vCPU.

SMCCC probe requires PSCI v1.x. If userspace only implements PSCI v0.2,
the guest won't query SMCCC support through PSCI and won't use the
spectre workarounds. We could hijack PSCI_VERSION and pretend to support
v1.0 if userspace does not, then handle all v1.0 calls ourselves
(including guessing the PSCI feature set implemented by the guest), but
that seems unnecessary. After all the API already allows userspace to
force a version lower than v1.0 using the firmware pseudo-registers.

The KVM_REG_ARM_PSCI_VERSION pseudo-register currently resets to either
v0.1 if userspace doesn't set KVM_ARM_VCPU_PSCI_0_2, or
KVM_ARM_PSCI_LATEST (1.0).

Suggested-by: James Morse <james.morse@arm.com>
Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
[morse: Added description of why this is required]
Signed-off-by: James Morse <james.morse@arm.com>