xen/9pfs: initialize len to 0 to detect xenbus_read errors

xen/9pfs: initialize len to 0 to detect xenbus_read errors

In order to use "len" to check for xenbus_read errors properly, we need
to initialize len to 0 before passing it to xenbus_read.

CC: [email protected]
CC: [email protected]
CC: [email protected]
CC: Eric Van Hensbergen <[email protected]>
CC: Ron Minnich <[email protected]>
CC: Latchesar Ionkov <[email protected]>
CC: [email protected]
Signed-off-by: Stefano Stabellini <[email protected]>
Reviewed-by: Juergen Gross <[email protected]>
Signed-off-by: Juergen Gross <[email protected]>