virt-host-validate: warn if kvm_hv is not loaded for POWER hosts

POWER hosts does not implement CPU virtualization extensions like
x86 or s390x. Instead, all bare-metal POWER hosts are considered
to be virtualization ready.

For POWER, the validation is done by checking if the virtualization
module kvm_hv is loaded in the host. If not, we should warn the
user about it.

Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>