util: virhostcpu: parse frequency information on S390
    
    Since kernel version 4.7, processor frequency information is available
    on S390. Let's adjust the parser so this information shows up for virsh
    nodeinfo:
    
        # virsh nodeinfo
        CPU model:           s390x
        CPU(s):              8
        CPU frequency:       5000 MHz
        CPU socket(s):       1
        Core(s) per socket:  8
        Thread(s) per core:  1
        NUMA cell(s):        1
        Memory size:         16273908 KiB
    
    Reviewed-by: Marc Hartmayer <mhartmay@linux.vnet.ibm.com>
    Reviewed-by: Boris Fiuczynski <fiuczy@linux.vnet.ibm.com>
    Signed-off-by: Bjoern Walk <bwalk@linux.vnet.ibm.com>