conf: storage: pool: reject name containing '/'

conf: storage: pool: reject name containing '/'

Trying to define a pool name containing an embedded '/'
will immediately fail when trying to write the XML to disk.
This patch explicitly rejects names containing a '/'

Besides our stateful driver, there are two other storage impls:
esx and phyp. esx doesn't support pool creation, so this should
doesn't apply.

phyp does support pool creation, and the name is passed to the
'mksp' tool, which google doesn't reveal whether it accepts '/'
or not. IMO the likeliness of this impacting any users is near zero