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;

vm-virtio/src/transport/mod.rs

@@ -2,8 +2,11 @@
 //
 // SPDX-License-Identifier: Apache-2.0
 
+#[cfg(feature = "pci_support")]
 mod pci_common_config;
+#[cfg(feature = "pci_support")]
 mod pci_device;
-
+#[cfg(feature = "pci_support")]
 pub use pci_common_config::VirtioPciCommonConfig;
+#[cfg(feature = "pci_support")]
 pub use pci_device::VirtioPciDevice;

vm-virtio/src/transport/pci_device.rs

@@ -7,6 +7,7 @@
 // SPDX-License-Identifier: Apache-2.0 AND BSD-3-Clause
 
 extern crate devices;
+#[cfg(feature = "pci_support")]
 extern crate pci;
 extern crate vm_allocator;
 extern crate vm_memory;