ci: Run clippy for each specific feature

The build is run against "--all-features", "pci,acpi", "pci" and "mmio"
separately. The clippy validation must be run against the same set of
features in order to validate the code is correct.

Because of these new checks, this commit includes multiple fixes
related to the errors generated when manually running the checks.

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

vm-virtio/src/lib.rs

@@ -130,6 +130,7 @@ impl fmt::Display for VirtioDeviceType {
 const INTERRUPT_STATUS_USED_RING: u32 = 0x1;
 #[allow(dead_code)]
 const INTERRUPT_STATUS_CONFIG_CHANGED: u32 = 0x2;
+#[cfg(feature = "pci_support")]
 const VIRTIO_MSI_NO_VECTOR: u16 = 0xffff;
 
 #[derive(Debug)]