Change xen_host_pci_sysfs_path() to return void

And assert the snprintf() error, because user can do nothing in case of
snprintf() fail.

Signed-off-by: Cao jin <caoj.fnst@cn.fujitsu.com>
Reviewed-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
