gtk: Fix -serial vc

Try kicking off a rhel5 text install over serial, the text menu navigation
is all messed up, and some of the kernel boot messages are randomly
corrupted.

Drop use of a pty and just use vte infrastructure for reading and writing.
This fixes the above corruption, and is simpler to boot.

(I don't know what was wrong with the original code though. FWIW this is
what virt-manager has done for years).

Signed-off-by: Cole Robinson <crobinso@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
