Show host model in domain capabilities

Show host model in domain capabilities

The domain capabilities XML is capable of showing whether each guest CPU
mode is supported or not with a possibility to provide additional
details. This patch enhances host-model capability to advertise the
exact CPU model which will be used as a host-model:

    <cpu>
        ...
        <mode name='host-model' supported='yes'>
            <model fallback='allow'>Broadwell</model>
            <vendor>Intel</vendor>
            <feature policy='disable' name='aes'/>
            <feature policy='require' name='vmx'/>
        </mode>
        ...
    </cpu>

Signed-off-by: Jiri Denemark <jdenemar@redhat.com>