revert to using normal rust-vmm-ci tests

Now that rust-vmm-ci uses cargo-all-features, we can deal with the
incompatiblity of the xen and postcopy features by simply marking them
as incompatible in Cargo.toml, instead of needing to define separate CI
steps.

note: this commit currently symlinks rust-vmm-ci-tests.json to
rust-vmm-ci/.buildkite/test_descriptions.json, and removes all tests
from custom-tests.json. This is to illustrate that the new setup works
without changes to the buildkite pipeline. Once that is confirmed, the
pipeline should be updated to just use the rust-vmm-ci test generaiton
routine, and the entire .buildkite directory can be removed.

Signed-off-by: Patrick Roy <[email protected]>

Author: roypat

.buildkite/custom-tests.json

@@ -1,25 +1,4 @@
 {
     "tests": [
-        {
-            "test_name": "build-vhost-kern",
-            "command": "cargo build --features=vhost-kern",
-            "platform": [
-                "x86_64"
-            ]
-        },
-        {
-            "test_name": "build-vhost-user-frontend",
-            "command": "cargo build --features=vhost-user-frontend",
-            "platform": [
-                "x86_64"
-            ]
-        },
-        {
-            "test_name": "build-vhost-user-backend",
-            "command": "cargo build --features=vhost-user-backend",
-            "platform": [
-                "x86_64"
-            ]
-        }
     ]
 }

.buildkite/rust-vmm-ci-tests.json

@@ -0,0 +1 @@
+../rust-vmm-ci/.buildkite/test_description.json