xen: xenbus: set error code on failure

Variable err is initialized with 0. As a result, the return value may
be 0 even if get_zeroed_page() fails to allocate memory. This patch fixes
the bug, initializing err with "-ENOMEM".

Signed-off-by: Pan Bian <bianpan2016@163.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Juergen Gross <jgross@suse.com>