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/Cargo.toml

@@ -4,6 +4,10 @@ version = "0.1.0"
 authors = ["Samuel Ortiz <sameo@linux.intel.com>"]
 edition = "2018"
 
+[features]
+default = []
+pci_support = ["pci"]
+
 [dependencies]
 byteorder = "1.3.2"
 devices = { path = "../devices" }
@@ -12,7 +16,7 @@ libc = "0.2.60"
 log = "0.4.8"
 net_gen = { path = "../net_gen" }
 net_util = { path = "../net_util" }
-pci = { path = "../pci" }
+pci = { path = "../pci", optional = true }
 tempfile = "3.1.0"
 virtio-bindings = { path = "../virtio-bindings" }
 vm-allocator = { path = "../vm-allocator" }