Use the new virDomainUpdateDeviceFlags API to allow the VNC password
to be changed on the fly

* src/internal.h: Define STREQ_NULLABLE() which is like STREQ()
  but does not crash if either argument is NULL, and treats two
  NULLs as equal.
* src/libvirt_private.syms: Export virDomainGraphicsTypeToString
* src/qemu/qemu_driver.c: Support VNC password change on a live
  machine
* src/qemu/qemu_monitor.c: Disable crazy debugging info. Treat a
  NULL password as "" (empty string), allowing passwords to be
  disabled in the monitor