KVM: arm64: Add KVM_ARM_VCPU_PMU_V3_SET_PMU attribute

When KVM creates an event and there are more than one PMUs present on the
system, perf_init_event() will go through the list of available PMUs and
will choose the first one that can create the event. The order of the PMUs
in the PMU list depends on the probe order, which can change under various
circumstances, for example if the order of the PMU nodes change in the DTB
or if asynchronous driver probing is enabled on the kernel command line
(with the driver_async_probe=armv8-pmu option).

Another consequence of this approach is that, on heteregeneous systems,
all virtual machines that KVM creates will use the same PMU. This might
cause unexpected behaviour for userspace: when a VCPU is executing on
the physical CPU that uses this PMU, PMU events in the guest work
correctly; but when the same VCPU executes on another CPU, PMU events in
the guest will suddenly stop counting.

Fortunately, perf core allows user to specify on which PMU to create an
event by using the perf_event_attr->type field, which is used by
perf_init_event() as an index in the radix tree of available PMUs.

Add the KVM_ARM_VCPU_PMU_V3_CTRL(KVM_ARM_VCPU_PMU_V3_SET_PMU) VCPU
attribute to allow userspace to specify the arm_pmu that KVM will use when
creating events for that VCPU. KVM will make no attempt to run the VCPU on
the physical CPUs that share this PMU, leaving it up to userspace to
manage the VCPU threads' affinity accordingly.

Setting the PMU for a VCPU is an all of nothing affair to avoid exposing an
asymmetric system to the guest: either all VCPUs have the same PMU, either
none of the VCPUs have a PMU set. Attempting to do something in between
will result in an error being returned when doing KVM_ARM_VCPU_PMU_V3_INIT.