Recently CPU hardware vendors have started to support a new structure
inside the CPU package topology known as a "die". Thus the hierarchy
is now:

  sockets > dies > cores > threads

This adds support for "dies" in the XML parser, with the value
defaulting to 1 if not specified for backwards compatibility.

For example a system with 64 logical CPUs might report

   <topology sockets="4" dies="2" cores="4" threads="2"/>
Reviewed-by: NJiri Denemark <jdenemar@redhat.com>
Reviewed-by: NDaniel Henrique Barboza <danielhb413@gmail.com>
Signed-off-by: NDaniel P. Berrangé <berrange@redhat.com>

While "x86" is a CPU sub driver name, it is not a recognized name of any
architecture known to libvirt. Let's use "x86_64" prefix which can be
used with virArch APIs.
Signed-off-by: NJiri Denemark <jdenemar@redhat.com>

The reworked API is now called virCPUUpdate and it should change the
provided CPU definition into a one which can be consumed by the QEMU
command line builder:

    - host-passthrough remains unchanged
    - host-model is turned into custom CPU with a model and features
      copied from host
    - custom CPU with minimum match is converted similarly to host-model
    - optional features are updated according to host's CPU
Signed-off-by: NJiri Denemark <jdenemar@redhat.com>

VIR_DOMAIN_XML_UPDATE_CPU flag for virDomainGetXMLDesc may be used to
get updated custom mode guest CPU definition in case it depends on host
CPU. This patch implements the same behavior for host-model and
host-passthrough CPU modes.

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.