arm64: Add --vcpu-affinity command line argument

Add a new command line argument, --vcpu-affinity, to set the CPU affinity
for the VCPUs. The affinity is expressed as a cpulist and will apply to all
VCPU threads.

This gives the user a second option for choosing the PMU on a heterogeneous
system. The PMU setup code, when --vcpu-affinity is specified, will search
for the PMU associated with the CPUs specified with this command line
argument instead of the PMU associated with the CPU on which the main
thread is executing.

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