pciif: add multi-vector-MSI command

The requested vector count is to be passed in struct xen_pci_op's info
field. Upon failure, if a smaller vector count might work, the backend
will pass that smaller count in the value field (which so far is always
being set to zero in the error path).

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/include/public/io/pciif.h
+++ b/xen/include/public/io/pciif.h
@@ -46,6 +46,7 @@
 #define XEN_PCI_OP_aer_resume		(7)
 #define XEN_PCI_OP_aer_mmio		(8)
 #define XEN_PCI_OP_aer_slotreset	(9)
+#define XEN_PCI_OP_enable_multi_msi	(10)
 
 /* xen_pci_op error numbers */
 #define XEN_PCI_ERR_success          (0)