ccid: add ccid-card-emulated device

This devices uses libcacard (internal) to emulate a smartcard conforming
to the CAC standard. It attaches to the usb-ccid bus. Usage instructions
(example command lines) are in the following patch in docs/ccid.txt. It
uses libcacard which uses nss, so it can work with both hw cards and
certificates (files).

Signed-off-by: Alon Levy <alevy@redhat.com>

---

changes from v20->v21: (Jes Sorenson review)
 * cosmetics
 * use qemu-thread and qemu_malloc/qemu_free

changes from v19->v20:
 * checkpatch.pl

changes from v18->v19:
 * add qdev.desc
 * backend: drop the enumeration property, back to using a string one.

changes from v16->v17:
 * use PROP_TYPE_ENUM for backend

changes from v15->v16:
 * fix error reporting in initfn
 * bump copyright year
 * update copyright license

changes from v1:
 * remove stale comments, use only c-style comments
 * bugfix, forgot to set recv_len
 * change reader name to 'Virtual Reader'
