scripts/qemu.py: support adding a console with the default serial device

scripts/qemu.py: support adding a console with the default serial device

The set_console() utility function either adds a device based on the
explicitly given device type, or adds a known good type of device
based on the machine type.

But, for a number of machine types, it may be impossible or
inconvenient to add the devices by means of "-device" command line
options, and then it may better to just use the "-serial" option and
let QEMU itself, based on the machine type, set the device
accordingly.

To achieve that, the behavior of set_console() now flags the intention
to add a console device on launch(), and if no explicit device type is
given the "-serial" option is going to be added to the QEMU command
line, instead of raising exceptions.

Based on testing with different machine types, the CONSOLE_DEV_TYPES
is not necessary anymore, so it's being removed, as is the logic to
use it.

Signed-off-by: Cleber Rosa <crosa@redhat.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Message-Id: <20190312171824.5134-13-crosa@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>