xci: Switch default CPU model to 'host-model'

xci: Switch default CPU model to 'host-model'

From upstream docs:

'The host-model mode is essentially a shortcut to copying host CPU
definition from capabilities XML into domain XML. Since the CPU
definition is copied just before starting a domain, exactly the
same XML can be used on different hosts while still providing
the best guest CPU each host supports'

It's probably safer to use that instead of 'host-passthrough' so
we can get a CPU (both for main VM and the nested ones) that libvirt
understands. Moreover, it's important to present a CPU that the
guest OS understands as well.

Change-Id: I25a8ff0e8635df9804c793d184f048cc86059ce0
Signed-off-by: Markos Chandras <mchandras@suse.de>