virt-host-validate: improve tests for arm/aarch64

virt-host-validate: improve tests for arm/aarch64

ARM/Aarch64 /proc/cpuinfo has no virtualization related flags.
Refactor the Qemu/KVM test a bit:

1) run the "for hardware virtualization" test only on plaforms with known
   cpuinfo flags (x86, s390)
2) test for /dev/kvm also on platforms where no cpu flags are set

Finally Add a more generic error hint message for non-x86 plaforms
when /dev/kvm is missing.

Signed-off-by: Riku Voipio <riku.voipio@linaro.org>
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>