pci: Improve MSI-X code to let VFIO rely on it

This commit enhances the current msi-x code hosted in the pci crate
in order to be reused by the vfio crate. Specifically, it creates
several useful methods for the MsixCap structure that can simplify
the caller's code.

Signed-off-by: Sebastien Boeuf <sebastien.boeuf@intel.com>

pci/src/lib.rs

@@ -24,7 +24,7 @@ pub use self::configuration::{
 pub use self::device::{
     Error as PciDeviceError, InterruptDelivery, InterruptParameters, PciDevice,
 };
-pub use self::msix::{MsixCap, MsixConfig, MsixTableEntry};
+pub use self::msix::{MsixCap, MsixConfig, MsixTableEntry, MSIX_TABLE_ENTRY_SIZE};
 
 /// PCI has four interrupt pins A->D.
 #[derive(Copy, Clone)]