ARM: KVM: iterate over all CPUs for CPU compatibility check

kvm_target_cpus() checks the compatibility of the used CPU with
KVM, which is currently limited to ARM Cortex-A15 cores.
However by calling it only once on any random CPU it assumes that
all cores are the same, which is not necessarily the case (for example
in Big.Little).

[ I cut some of the commit message and changed the formatting of the
  code slightly to pass checkpatch and look more like the rest of the
  kvm/arm init code - Christoffer ]

Signed-off-by: Andre Przywara <andre.przywara@linaro.org>
Signed-off-by: Christoffer Dall <cdall@cs.columbia.edu>