Sanitise the xenstore path elements (no '.' for example)
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 <63c9eb0ea83039690fefa11afe17873ba8278a56@eu.citrix.com>

mirage/xen-disk