Introduce migration cookies to QEMU driver

Introduce migration cookies to QEMU driver

The migration protocol has support for a 'cookie' parameter which
is an opaque array of bytes as far as libvirt is concerned. Drivers
may use this for passing around arbitrary extra data they might
need during migration. The QEMU driver needs to do a few things:

 - Pass hostname/uuid to allow strict protection against localhost
   migration attempts
 - Pass SPICE/VNC server port from the target back to the source to
   allow seamless relocation of client sessions
 - Pass lock driver state from source to destination

This patch introduces the basic glue for handling cookies
but only includes the host/guest UUID & name.

* src/libvirt_private.syms: Export virXMLParseStrHelper
* src/qemu/qemu_migration.c, src/qemu/qemu_migration.h: Parsing
  and formatting of migration cookies
* src/qemu/qemu_driver.c: Pass in cookie parameters where possible
* src/remote/remote_protocol.h, src/remote/remote_protocol.x: Change
  cookie max length to 16384 bytes