Define XML syntax for password expiry

Define XML syntax for password expiry

This extends the XML syntax for <graphics> to allow a password
expiry time to be set

eg

  <graphics type='vnc' port='5900' autoport='yes' keymap='en-us' passwd='12345' passwdValidTo='2010-04-09T15:51:00'/>

The timestamp is in UTC.

* src/conf/domain_conf.h: Pull passwd out into separate struct
  virDomainGraphicsAuthDef to allow sharing between VNC & SPICE
* src/conf/domain_conf.c: Add parsing/formatting of new passwdValidTo
  argument
* src/opennebula/one_conf.c, src/qemu/qemu_conf.c, src/qemu/qemu_driver.c,
  src/xen/xend_internal.c, src/xen/xm_internal.c: Update for changed
  struct containing VNC password