Currently the virConnectBaselineCPU API does not expose the CPU features
that are part of the CPU's model.  This patch adds a new flag,
VIR_CONNECT_BASELINE_CPU_EXPAND_FEATURES, that causes the API to explicitly
list all features that are part of that model.
Signed-off-by: NDon Dugger <donald.d.dugger@intel.com>
Signed-off-by: NEric Blake <eblake@redhat.com>

The mode can be either of "custom" (default), "host-model",
"host-passthrough". The semantics of each mode is described in the
following examples:

- guest CPU is a default model with specified topology:
    <cpu>
      <topology sockets='1' cores='2' threads='1'/>
    </cpu>

- guest CPU matches selected model:
    <cpu mode='custom' match='exact'>
      <model>core2duo</model>
    </cpu>

- guest CPU should be a copy of host CPU as advertised by capabilities
  XML (this is a short cut for manually copying host CPU specification
  from capabilities to domain XML):
    <cpu mode='host-model'/>

  In case a hypervisor does not support the exact host model, libvirt
  automatically falls back to a closest supported CPU model and
  removes/adds features to match host. This behavior can be disabled by
    <cpu mode='host-model'>
      <model fallback='forbid'/>
    </cpu>

- the same as previous returned by virDomainGetXMLDesc with
  VIR_DOMAIN_XML_UPDATE_CPU flag:
    <cpu mode='host-model' match='exact'>
      <model fallback='allow'>Penryn</model>       --+
      <vendor>Intel</vendor>                         |
      <topology sockets='2' cores='4' threads='1'/>  + copied from
      <feature policy='require' name='dca'/>         | capabilities XML
      <feature policy='require' name='xtpr'/>        |
      ...                                          --+
    </cpu>

- guest CPU should be exactly the same as host CPU even in the aspects
  libvirt doesn't model (such domain cannot be migrated unless both
  hosts contain exactly the same CPUs):
    <cpu mode='host-passthrough'/>

- the same as previous returned by virDomainGetXMLDesc with
  VIR_DOMAIN_XML_UPDATE_CPU flag:
    <cpu mode='host-passthrough' match='minimal'>
      <model>Penryn</model>                        --+ copied from caps
      <vendor>Intel</vendor>                         | XML but doesn't
      <topology sockets='2' cores='4' threads='1'/>  | describe all
      <feature policy='require' name='dca'/>         | aspects of the
      <feature policy='require' name='xtpr'/>        | actual guest CPU
      ...                                          --+
    </cpu>

In case a hypervisor doesn't support the exact CPU model requested by a
domain XML, we automatically fallback to a closest CPU model the
hypervisor supports (and make sure we add/remove any additional features
if needed). This patch adds 'fallback' attribute to model element, which
can be used to disable this automatic fallback.