build: make PCI (virtio and vfio) disableable at build time

Although included by default it is now possible to build without PCI
support.

Signed-off-by: Rob Bradford <robert.bradford@intel.com>

vm-virtio/src/lib.rs

@@ -12,6 +12,7 @@
 extern crate epoll;
 #[macro_use]
 extern crate log;
+#[cfg(feature = "pci_support")]
 extern crate pci;
 extern crate vhost_rs;
 extern crate virtio_bindings;