pci/xen: Use xen_allocate_pirq_msi instead of xen_allocate_pirq

xen_allocate_pirq -> xen_map_pirq_gsi -> PHYSDEVOP_alloc_irq_vector IFF
xen_initial_domain() in addition to the kernel side book-keeping side of
things (set chip and handler, update irq_info etc) whereas
xen_allocate_pirq_msi just does the kernel book keeping.

Also xen_allocate_pirq allocates an IRQ in the 1-1 GSI space whereas
xen_allocate_pirq_msi allocates a dynamic one in the >GSI IRQ space.

All of this is uneccessary as this code path is only executed
when we run as a domU PV guest with an MSI/MSI-X PCI card passed in.
Hence we can jump straight to allocating an dynamic IRQ (and
binding it to the proper PIRQ) and skip the rest.

In short: this change is a cosmetic one.
Reviewed-by: Ian Campbell <Ian.Campbell@citrix.com>
Reviewed-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

arch/x86/pci/xen.c

@@ -157,14 +157,14 @@ static int xen_setup_msi_irqs(struct pci_dev *dev, int nvec, int type)
 		goto error;
 	i = 0;
 	list_for_each_entry(msidesc, &dev->msi_list, list) {
-		irq = xen_allocate_pirq(v[i], 0, /* not sharable */
+		xen_allocate_pirq_msi(
 			(type == PCI_CAP_ID_MSIX) ?
-			"pcifront-msi-x" : "pcifront-msi");
+			"pcifront-msi-x" : "pcifront-msi",
+			&irq, &v[i], XEN_ALLOC_IRQ);
 		if (irq < 0) {
 			ret = -1;
 			goto free;
 		}
-
 		ret = set_irq_msi(irq, msidesc);
 		if (ret)
 			goto error_while;