bootdevice: Refactor get_boot_devices_list

Move device name construction to a separate function.

We will reuse this function in the following commit to pass logical CHS
parameters through fw_cfg much like we currently pass bootindex.

Reviewed-by: Karl Heubaum <karl.heubaum@oracle.com>
Reviewed-by: Arbel Moshe <arbel.moshe@oracle.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Sam Eiderman <shmuel.eiderman@oracle.com>
Signed-off-by: Sam Eiderman <sameid@google.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: John Snow <jsnow@redhat.com>