conf: add deprecated_features attribute

conf: add deprecated_features attribute

Add a new a attribute, deprecated_features='on|off' to the <cpu>
element.  This is used to toggle features flagged as deprecated on the
CPU model on or off.  When this attribute is paired with 'on',
deprecated features will not be filtered.  When paired with 'off', any
CPU features that are flagged as deprecated will be listed under the
CPU model with the 'disable' policy.

Example:

  <cpu mode='host-model' check='partial' deprecated_features='off'/>

The absence of this attribute is equivalent to the 'on' option.

The deprecated features that will populate the domain XML are the same
features that result in the virsh domcapabilities command with the
--disable-deprecated-features argument present.

It is recommended to define a domain XML with this attribute set to
'off' to ensure migration to machines that may outright drop these
features in the future.

Signed-off-by: Collin Walling <walling@linux.ibm.com>
Reviewed-by: Jiri Denemark <jdenemar@redhat.com>