xe: make script more robust

  - add 'set -e' to stop on error
  - print the VDI, VM uuid (since the names aren't guaranteed to be unique)

Signed-off-by: David Scott <dave.scott@citrix.com>
xe: store the VIF uuid in a $VAR

  - avoids unnecessarily writing the bare UUID to the console
  - restores symmetry with VBD handling

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