9p/xen: fix check for xenbus_read error in front_probe

If the xen bus exists but does not expose the proper interface, it is
possible to get a non-zero length but still some error, leading to
strcmp failing trying to load invalid memory addresses e.g.
fffffffffffffffe.

There is then no need to check length when there is no error, as the
xenbus driver guarantees that the string is nul-terminated.

Link: http://lkml.kernel.org/r/1534236007-10170-1-git-send-email-asmadeus@codewreck.org
Signed-off-by: Dominique Martinet <dominique.martinet@cea.fr>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Cc: Eric Van Hensbergen <ericvh@gmail.com>
Cc: Latchesar Ionkov <lucho@ionkov.net>