target-i386: Add "feature-words" property to X86CPU

This property will be useful for libvirt, as libvirt already has logic
based on low-level feature bits (not feature names), so it will be
really easy to convert the current libvirt logic to something using the
"feature-words" property.

The property will have two main use cases:
 - Checking host capabilities, by checking the features of the "host"
   CPU model
 - Checking which features are enabled on each CPU model

Example output:

  $ ./QMP/qmp --path=/tmp/m \
    qom-get --path=/machine/icc-bridge/icc/child[0] \
            --property=feature-words
  item[0].cpuid-register: EDX
  item[0].cpuid-input-eax: 2147483658
  item[0].features: 0
  item[1].cpuid-register: EAX
  item[1].cpuid-input-eax: 1073741825
  item[1].features: 0
  item[2].cpuid-register: EDX
  item[2].cpuid-input-eax: 3221225473
  item[2].features: 0
  item[3].cpuid-register: ECX
  item[3].cpuid-input-eax: 2147483649
  item[3].features: 101
  item[4].cpuid-register: EDX
  item[4].cpuid-input-eax: 2147483649
  item[4].features: 563346425
  item[5].cpuid-register: EBX
  item[5].cpuid-input-eax: 7
  item[5].features: 0
  item[5].cpuid-input-ecx: 0
  item[6].cpuid-register: ECX
  item[6].cpuid-input-eax: 1
  item[6].features: 2155880449
  item[7].cpuid-register: EDX
  item[7].cpuid-input-eax: 1
  item[7].features: 126614521

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>