ixgbed: use MSI-X (via pci_allocate_interrupt_vector) when available

The previous code unconditionally required a legacy INTX line, which
on bare metal is missing for nearly every modern 82599/X540/X550
board that has its IRQ lines wired to MSI-X only. pcid_interface
allocates MSI-X when the device advertises it, falls back to MSI,
then to legacy INTX, so the change preserves every existing path
while unlocking the much higher line-rate headroom of MSI-X
per-vector queuing that the device's enable_msix_interrupt code
already implements in device.rs.

The IRQ-acknowledge-and-write pattern is unchanged: the kernel
masks the line on delivery, we must re-arm unconditionally or a
foreign interrupt on a shared line leaves the line masked forever.

drivers/net/ixgbed/src/main.rs

@@ -19,10 +19,14 @@ fn daemon(daemon: daemon::Daemon, mut pcid_handle: PciFunctionHandle) -> ! {
     let mut name = pci_config.func.name();
     name.push_str("_ixgbe");
 
+    // Allocate one interrupt vector. pcid_interface::pci_allocate_interrupt_vector
+    // prefers MSI-X when the device advertises it and falls back to MSI and
+    // legacy INTX otherwise, so this single line makes the driver work on
+    // every modern 82599/X540/X550 board that has MSI-X wired.
     let irq = pci_config
         .func
-        .legacy_interrupt_line
-        .expect("ixgbed: no legacy interrupts supported");
+        .pci_allocate_interrupt_vector(1)
+        .expect("ixgbed: failed to allocate an interrupt vector");
 
     println!(" + IXGBE {}", pci_config.func.display());
 
@@ -74,9 +78,6 @@ fn daemon(daemon: daemon::Daemon, mut pcid_handle: PciFunctionHandle) -> ! {
                 let mut irq = [0; 8];
                 irq_file.read(&mut irq).unwrap();
                 let ours = scheme.adapter().irq();
-                // The kernel masks the IRQ line on delivery and only re-arms
-                // it on write-back; ack unconditionally or a foreign interrupt
-                // on a shared line leaves the line masked forever.
                 irq_file.write(&mut irq).unwrap();
 
                 if ours {