avocado_qemu: add exec_command function

avocado_qemu: add exec_command function

Sometimes a test needs to send a command to a console without waiting
for a pattern as a result, or the command issued do not produce any kind
of output, like, for example, a `mount` command.

This introduces the `exec_command` function to the avocado_qemu,
allowing the test to send a command to the console without the need to
match a pattern produced as a result.

Signed-off-by: Willian Rampazzo <willianr@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Wainer dos Santos Moschetta <wainersm@redhat.com>
Reviewed-by: Cleber Rosa <crosa@redhat.com>
Tested-by: Cleber Rosa <crosa@redhat.com>
Message-Id: <20210303205320.146047-2-willianr@redhat.com>
Signed-off-by: Cleber Rosa <crosa@redhat.com>