Fix for bnc #890881

 - Read the uuid generated by the hypervisor if it exists
 + On paravirtual Xen guests there is no dmi information, thus dmidecode
    fails. Instead a UUID is generated by the hypervisor and made available
    in /sys/hypervisor/uuid use this value for registration.
  + This allows us to register guests running in EC2 and
    addresses bnc #890881
readable? checks the user permissions but we have to check the presence too