xen/pciback: Fix error return code in xen_pcibk_attach()

Fix to return -EFAULT from the error handling case instead of 0 when
version mismatch with pcifront.

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Reviewed-by Jan Beulich <jbeulich@suse.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>