xen-pciback: Use pci_enable_msix_exact() instead of pci_enable_msix()

As result of deprecation of MSI-X/MSI enablement functions
pci_enable_msix() and pci_enable_msi_block() all drivers
using these two interfaces need to be updated to use the
new pci_enable_msi_range() or pci_enable_msi_exact()
and pci_enable_msix_range() or pci_enable_msix_exact()
interfaces.

Signed-off-by: Alexander Gordeev <[e-mail]>
Cc: Konrad Rzeszutek Wilk <[e-mail]>
Cc: Boris Ostrovsky <[e-mail]>
Cc: David Vrabel <[e-mail]>
Cc: [e-mail]
Signed-off-by: Konrad Rzeszutek Wilk <[e-mail]>
Reviewed-by: Boris Ostrovsky <[e-mail]>

drivers/xen/xen-pciback/pciback_ops.c

···
 		entries[i].vector = op->msix_entries[i].vector;
 	}
 
-	result = pci_enable_msix(dev, entries, op->value);
-
+	result = pci_enable_msix_exact(dev, entries, op->value);
 	if (result == 0) {
 		for (i = 0; i < op->value; i++) {
 			op->msix_entries[i].entry = entries[i].entry;