Maintain a list of QemuVT100 instances so the cursor timer can directly
iterate over them and call vt100_refresh(), instead of going through
qemu_invalidate_text_consoles() which iterated over all consoles
(including graphic ones) and called back into the generic display layer.
This removes the qemu_invalidate_text_consoles() function from
console.c, further decoupling VT100 text rendering from the console
core.
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>