libxl: remove xenstore /local/domain/<domid> after reading /vm and /vss paths

libxl: remove xenstore /local/domain/<domid> after reading /vm and /vss paths

On domain destroy libxl currently tries to find the /vm/<uuid> and
/vss/<uuid> paths by reading /local/domain/<domid>/{vm,vss} but does
so after removing /local/domain/<domid> which isn't much use.

Also construct the xenstore path in destroy the same way as in create. It's not
wrong but I had to look twice to check they were doing the same thing.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>