virsh: Add --model option for hypervisor-cpu-baseline

virsh: Add --model option for hypervisor-cpu-baseline

This option can be used as a shortcut for creating a single XML with
just a CPU model name and no features:

    $ virsh hypervisor-cpu-baseline --model Skylake-Server
    <cpu mode='custom' match='exact'>
      <model fallback='forbid'>Skylake-Server</model>
      <feature policy='disable' name='avx512f'/>
      <feature policy='disable' name='avx512dq'/>
      <feature policy='disable' name='clwb'/>
      <feature policy='disable' name='avx512cd'/>
      <feature policy='disable' name='avx512bw'/>
      <feature policy='disable' name='avx512vl'/>
      <feature policy='disable' name='pku'/>
    </cpu>

Signed-off-by: Jiri Denemark <jdenemar@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>