xen: set error code on failures

Variable rc is reset in the loop, and its value will be non-negative
during the second and after repeat of the loop. If it fails to allocate
memory then, it may return a non-negative integer, which indicates no
error. This patch fixes the bug, assigning "-ENOMEM" to rc when
kzalloc() or alloc_page() returns NULL, and removing the initialization
of rc outside of the loop.

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