webadmin: Added POSIX VFS validation & hint on storage create

Added a validation of special characters to a VFS type when creating
a POSIX domain to allow only ascii characters, also added a tooltip hint
on the input field to explain and show common types

Change-Id: I6a34d269da73f1b42b2b2917469f8ea337ca51ee
Signed-off-by: Tal Nisan <tnisan@redhat.com>