default devices: core code & serial lines.

Qemu creates a default serial line for you in case you didn't specify
one on the command line.  Right now this is tied to the '-serial
<chardev>' command line switch, which in turn causes trouble if you are
creating your serial line via '-device isa-serial,<props>'.

This patch adds a variable default_serial which says whenever a default
serial line should be added.  It is enabled by default.  It is cleared
when qemu finds '-serial' or '-device isa-serial' on the command line.

Part of the patch is some infrastructure for the '-device $driver'
checking (default_driver_check function) which will also be used by the
other patches of this series.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
