arm64: Add support for KVM_ARM_VCPU_PMU_V3_SET_PMU

The KVM_ARM_VCPU_PMU_V3_CTRL(KVM_ARM_VCPU_PMU_V3_SET_PMU) VCPU ioctl is
used to assign a physical PMU to the events that KVM creates when emulating
the PMU for that VCPU. This is useful on heterogeneous systems, when there
is more than one hardware PMU present.

The assumption that is made in the implementation is that the user will
pin the kvmtool process on a set of CPUs that share the same PMU. This
allows kvmtool to set the same PMU for all VCPUs from the main thread,
instead of in the individual VCPU threads.

Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>