Fix squeezed following semantic change in xs api

Turns out the old xenstore client would silently ignore Xb.Noent
exceptions raised by 'rm', but the new xenstore client doesn't.

Not entirely sure whether this should be changed in the new
xenstore client - it's likely to lead to some regressions if
we're not very careful.

Signed-off-by: Jon Ludlam <jonathan.ludlam@eu.citrix.com>