tests/boot_linux_console: rename the x86_64 after the arch and machine

tests/boot_linux_console: rename the x86_64 after the arch and machine

Given that the test is specific to x86_64 and pc, and new tests are
going to be added to the same class, let's rename it accordingly.
Also, let's make the class documentation not architecture specific.

Signed-off-by: Cleber Rosa <[email protected]>
Reviewed-by: Caio Carrara <[email protected]>
Reviewed-by: Philippe Mathieu-Daudé <[email protected]>
Reviewed-by: Cornelia Huck <[email protected]>
Message-Id: <20190312171824[email protected]>
Signed-off-by: Eduardo Habkost <[email protected]>