Acceptance tests: add Linux kernel boot and console checking test

Acceptance tests: add Linux kernel boot and console checking test

This test boots a Linux kernel, and checks that the given command
line was effective in two ways:

 * It makes the kernel use the set "console device" as a console
 * The kernel records the command line as expected in the console

Given that way too many error conditions may occur, and detecting the
kernel boot progress status may not be trivial, this test relies on a
timeout to handle unexpected situations.  Also, it's *not* tagged as a
quick test for obvious reasons.

It may be useful, while interactively running/debugging this test, or
tests similar to this one, to show some of the logging channels.
Example:

 $ avocado --show=QMP,console run boot_linux_console.py

Signed-off-by: Cleber Rosa <crosa@redhat.com>
Message-Id: <20180530184156.15634-6-crosa@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>