[PYGRUB] Plumb bootloader I/O through xenconsole.

[PYGRUB] Plumb bootloader I/O through xenconsole.

 - xend forwards console traffic between the running bootloader and a pty
   which it writes to the store so the xenconsole client can see it.
 - the xenconsole client handles the domain's console pty changing.
 - xm create no longer runs the bootloader.
 - pygrub gets '-i' option to explicitly request an interactive session.
 - xend unlocks the domain list during bootloading so that "xm console"
   can see the domain and attach to its console pty.

Signed-off-by: Tim Deegan <Tim.Deegan@xensource.com>