[Zion][Denali] Move to MSI for AHCI devices.

This will allow us to properly do interrupts for XHCI devices in the
future.

Also move PCI device header parsing to a shared library.

Get rid of the old irq register format which supplied an irq number and
instead pass the appropriate irq number back out to the caller.

rust/lib/mammoth/src/mem.rs

@@ -120,6 +120,12 @@ impl<T> AsRef<T> for MemoryRegion {
     }
 }
 
+impl<T> AsMut<T> for MemoryRegion {
+    fn as_mut(&mut self) -> &mut T {
+        unsafe { (self.virt_addr as *mut T).as_mut().unwrap() }
+    }
+}
+
 impl Drop for MemoryRegion {
     fn drop(&mut self) {
         // FIXME: We shouldn't have to do this manual adjustment.