9p/xen: fix version parsing

When connecting the Xen 9pfs frontend to the backend, the "versions"
Xenstore entry written by the backend is parsed in a wrong way.

The "versions" entry is defined to contain the versions supported by
the backend separated by commas (e.g. "1,2"). Today only version "1"
is defined. Unfortunately the frontend doesn't look for "1" being
listed in the entry, but it is expecting the entry to have the value
"1".

This will result in failure as soon as the backend will support e.g.
versions "1" and "2".

Fix that by scanning the entry correctly.

Link: https://lkml.kernel.org/r/20230130113036.7087-2-jgross@suse.com


Fixes: 71ebd719 ("xen/9pfs: connect to the backend")
Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>
Signed-off-by: Eric Van Hensbergen <ericvh@kernel.org>