console: minimal hotplug suport

console: minimal hotplug suport

This patch allows to unbind devices from QemuConsoles, using the new
graphic_console_close() function.  The QemuConsole will show a static
display then, saying the device was unplugged.  When re-plugging a
display later on the QemuConsole will be reused.

Eventually we will allocate and release QemuConsoles dynamically at some
point in the future, that'll need more infrastructure though to notify
user interfaces (gtk, sdl, spice, ...) about QemuConsoles coming and
going.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>