xen/platform-pci: use define instead of literal number

Instead of "0x01" use the HVM_PARAM_CALLBACK_TYPE_PCI_INTX define from
the interface header in get_callback_via().

Signed-off-by: Juergen Gross <jgross@suse.com>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>
Signed-off-by: Juergen Gross <jgross@suse.com>