vnc, xen: write vnc address and password to xenstore

vnc, xen: write vnc address and password to xenstore

The xend protocol as actually implemented is:
  * xend writes:
     /vm/UUID/vncpasswd = "PASS"   (n0,rDOMID)
     /local/domain/0/backend/vfb/DOMID/0/vncunused = "0"   (n0,rDOMID)
     /local/domain/0/backend/vfb/DOMID/0/vnc = "1"   (n0,rDOMID)
     /local/domain/0/backend/vfb/DOMID/0/vnclisten = "ADDR"   (n0,rDOMID)
     /local/domain/0/backend/vfb/DOMID/0/vncdisplay = "PORT"   (n0,rDOMID)
     /local/domain/0/backend/vfb/DOMID/0/vncpasswd = "PASS"   (n0,rDOMID)
  * qemu reads /vm/UUID/vncpasswd and overwrites it with "\0"
  * qemu writes
     /local/domain/DOMID/console/vnc-port = "PORT"   (n0,rDOMID)
  * xm vncviewer reads entries from backend/vfb,
    as well as console/vnc-port.
Much of this is insane.

xl quite properly does not create anything in backend/vfb for an HVM
domain with no vfb.  But xl vncviewer needs to know the port number
and the address and the password.

So, for now, have qemu write these nodes too:
     /local/domain/DOMID/console/vnc-listen = "ADDR"   (n0,rDOMID)
     /local/domain/DOMID/console/vnc-pass = "PASS"   (n0,rDOMID)
This corresponds to the protocol actually currently implemented in
libxl.

We will revisit this after the 4.1 release and invent a non-insane
protocol.

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>