smartcard: add XML support for <smartcard> device

smartcard: add XML support for <smartcard> device

Assuming a hypervisor that supports multiple smartcard devices in the
guest, this would be a valid XML description:

<devices>
  <smartcard mode='host'/>
  <smartcard mode='host-certificates'>
    <certificate>/path/to/cert1</certificate>
    <certificate>/path/to/cert2</certificate>
    <certificate>/path/to/cert3</certificate>
  </smartcard>
  <smartcard mode='passthrough' type='tcp'>
    <source mode='bind' host='127.0.0.1' service='2001'/>
    <protocol type='raw'/>
  </smartcard>
</devices>

(As of this commit, the qemu hypervisor will be the first
implementation, but it only supports one smartcard.)

* docs/formatdomain.html.in (Smartcard devices): New section.
* docs/schemas/domain.rng (smartcard): New define, used in
devices.
* tests/qemuxml2argvdata/qemuxml2argv-smartcard-host.xml: New file
to test schema.
* tests/qemuxml2argvdata/qemuxml2argv-smartcard-host-certificates.xml:
Likewise.
* tests/qemuxml2argvdata/qemuxml2argv-smartcard-passthrough-tcp.xml:
Likewise.
* tests/qemuxml2argvdata/qemuxml2argv-smartcard-controller.xml:
Likewise.