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>

arch/src/lib.rs

@@ -13,6 +13,7 @@ extern crate byteorder;
 extern crate kvm_bindings;
 extern crate libc;
 
+#[cfg(feature = "acpi")]
 extern crate acpi_tables;
 extern crate arch_gen;
 extern crate kvm_ioctls;