api_server: add unit tests to machine_configuration.rs
api_server: add unit tests to machine_configuration.rs

coverage: 95.2%

Signed-off-by: Alexandra Ghecenco <750ec4522de000f31ad44d64614ad5747f238032@amazon.com>

firecracker-microvm/firecracker,firecracker-microvm/firecracker,firecracker-microvm/firecracker,firecracker-microvm/firecracker,firecracker-microvm/firecracker