domcaps: Add support for listing supported CPU models

domcaps: Add support for listing supported CPU models

The patch adds <cpu> element to domain capabilities XML:

    <cpu>
        <mode name='host-passthrough' supported='yes'/>
        <mode name='host-model' supported='yes'/>
        <mode name='custom' supported='yes'>
            <model>Broadwell</model>
            <model>Broadwell-noTSX</model>
            ...
        </mode>
    </cpu>

Applications can use it to inspect what CPU configuration modes are
supported for a specific combination of domain type, emulator binary,
guest architecture and machine type.

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