qemu: Add support for setting TSC frequency

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>