libxl: create /local/domain/<domid>/error and make guest-writable

Frontends write error information to these paths which is currently
blocked with an EACCES. Typical errors from a Linux 3.x look like:

$ sudo xenstore-ls /local/domain/11/error -p
device = ""  . . . . . . . . . . . . . . . . . . . . . . . .  (n11)
 vbd = ""  . . . . . . . . . . . . . . . . . . . . . . . . .  (n11)
  5632 = ""  . . . . . . . . . . . . . . . . . . . . . . . .  (n11)
   error = "19 xenbus_dev_probe on device/vbd/5632"  . . . .  (n11)

Signed-off-by: David Scott <dave.scott@citrix.com>