qemu: Add support for setting TSC frequency
    
    QEMU allows for TSC frequency to be explicitly set to enable migration
    with invtsc (migration fails if the destination QEMU cannot set the
    exact same frequency used when starting the domain on the source host).
    
    Libvirt already supports setting the TSC frequency in the XML using
    
        <clock>
          <timer name='tsc' frequency='1234567890'/>
        </clock>
    
    which will be transformed into
    
        -cpu Model,tsc-frequency=1234567890
    
    QEMU command line.
    
    Signed-off-by: Jiri Denemark <jdenemar@redhat.com>