Sanitise the xenstore path elements (no '.' for example)

We use our argv.(0) to form the xenstore path. This can contain
illegal characters to be conservative and sanitise.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
The VM-name-or-uuid argument is mandatory.

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