build: Really make the acpi feature disableable

The command "cargo build --no-default-features" does not recursively
disable the default features across the workspace. Instead add an acpi
feature at the top-level, making it default, and then make that feature
conditional on all the crate acpi features.

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

Cargo.toml

@@ -18,7 +18,8 @@ tempdir= "0.3.7"
 lazy_static= "1.4.0"
 
 [features]
-default = []
+default = ["acpi"]
+acpi = ["vmm/acpi"]
 
 # Integration tests require a special environment to run in
 integration_tests = []