tests/boot_linux_console: add a test for alpha + clipper

tests/boot_linux_console: add a test for alpha + clipper

Similar to the x86_64 + pc test, it boots a Linux kernel on a Malta
board and verify the serial is working.  One extra command added to
the QEMU command line is '-vga std', because the kernel used is
known to crash without it.

If alpha is a target being built, "make check-acceptance" will
automatically include this test by the use of the "arch:alpha" tags.

Alternatively, this test can be run using:

    $ avocado run -t arch:alpha tests/acceptance
    $ avocado run -t machine:clipper tests/acceptance

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Cleber Rosa <crosa@redhat.com>
Reviewed-by: Caio Carrara <ccarrara@redhat.com>
Message-Id: <20190312171824.5134-21-crosa@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>