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>

vm-virtio/src/transport/pci_device.rs

@@ -354,6 +354,7 @@ impl VirtioPciDevice {
                 settings_bar,
                 self.msix_num,
                 MSIX_TABLE_BAR_OFFSET as u32,
+                settings_bar,
                 MSIX_PBA_BAR_OFFSET as u32,
             );
             self.configuration
@@ -401,7 +402,7 @@ impl PciDevice for VirtioPciDevice {
                 // device should not inject the interrupt.
                 // Instead, the Pending Bit Array table is updated to reflect there
                 // is a pending interrupt for this specific vector.
-                if config.is_masked() || entry.is_masked() {
+                if config.masked() || entry.masked() {
                     config.set_pba_bit(queue.vector, false);
                     return Ok(());
                 }