libcacard: improve documentation

libcacard: improve documentation

Using the file-backed smartcard backend is black magic, but it can
be useful if your only smartcard bricks itself if it is accessed
the wrong way too many times.

Complete the documentation to include the art of creating certificates
and using them with QEMU, based on Ray Strode's useful tutorial at
https://blogs.gnome.org/halfline/2013/09/08/another-smartcard-post/
but with ccid-card-emulated or vscclient instead of SPICE.

Cc: Ray Strode <rstrode@redhat.com>
Reviewed-by: Alon Levy <alevy@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>