xen/pciback: use resource_size()

 Use resource_size function on resource object
 instead of explicit computation.

 The semantic patch that makes this output is available
 in scripts/coccinelle/api/resource_size.cocci.

 More information about semantic patching is available at
 http://coccinelle.lip6.fr/



Signed-off-by: Thomas Meyer <thomas@m3y3r.de>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>