gtk: fix uninitialized temporary VirtualConsole

Only the echo field is used in the temporary VirtualConsole, so the
damage was limited.  But still, if echo was incorrectly set to true,
the result would be some puzzling output in VTE monitor and serial
consoles.

Fixes: fba958c692e47a373d15c1fd3d72b255bf76adbd
Cc: Gerd Hoffmann <kraxel@redhat.com>
Tested-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-id: 1455015557-15106-2-git-send-email-pbonzini@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
