vfio_device: make buildkite happy

Rust does not support mutually exclusive features. Buildkite enables all
features. These two factors combined causes pipeline failures.

Turn the feature gates in code a bit. Whenever "kvm" is specify, surface
KVM's definitions. Make available MSHV if and only if "mshv" is
specified.

Add a custom test for building on MSHV.

Signed-off-by: Wei Liu <liuwe@microsoft.com>

.buildkite/custom-tests.json

@@ -0,0 +1,9 @@
+{
+  "tests": [
+    {
+      "test_name": "build-mshv",
+      "command": "cargo build --release --no-default-features --features mshv",
+      "platform": ["x86_64"]
+    }
+  ]
+}

src/vfio_device.rs

@@ -22,11 +22,11 @@ use kvm_bindings::{
 #[cfg(feature = "kvm")]
 use kvm_ioctls::DeviceFd;
 use log::{debug, error, warn};
-#[cfg(feature = "mshv")]
+#[cfg(all(feature = "mshv", not(feature = "kvm")))]
 use mshv_bindings::{
     mshv_device_attr, MSHV_DEV_VFIO_GROUP, MSHV_DEV_VFIO_GROUP_ADD, MSHV_DEV_VFIO_GROUP_DEL,
 };
-#[cfg(feature = "mshv")]
+#[cfg(all(feature = "mshv", not(feature = "kvm")))]
 use mshv_ioctls::DeviceFd;
 use vfio_bindings::bindings::vfio::*;
 use vm_memory::{Address, GuestMemory, GuestMemoryRegion, MemoryRegionAddress};
@@ -243,7 +243,7 @@ impl VfioContainer {
             addr: group_fd_ptr as u64,
         };
 
-        #[cfg(feature = "mshv")]
+        #[cfg(all(feature = "mshv", not(feature = "kvm")))]
         let dev_attr = mshv_device_attr {
             flags: 0,
             group: MSHV_DEV_VFIO_GROUP,
@@ -266,7 +266,7 @@ impl VfioContainer {
             addr: group_fd_ptr as u64,
         };
 
-        #[cfg(feature = "mshv")]
+        #[cfg(all(feature = "mshv", not(feature = "kvm")))]
         let dev_attr = mshv_device_attr {
             flags: 0,
             group: MSHV_DEV_VFIO_GROUP,