Creating a domain with an invalid controller specification for a pvusb
device will currently segfault.
Avoid this by bailing out early in case of a mandatory xenstore path
not existing.
Signed-of-by: Juergen Gross <jgross@suse.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>