GTK3 interface doesn't zoom-to-fit by default

The SDL interface automatically scales the video output to
match the window size.  The GTK3 interface has an off-by-default option
"Zoom To Fit" for that.  As far as I can tell, no command-line option
exists to turn that option on.  That makes it harder to quickly zoom a
freshly launched VM; instead of just hitting a maximize-window hotkey, I
also have to navigate through the menu to select "Zoom To Fit".

Given that VMs typically start out running in a much lower-resolution
video mode than the host (and VMs not running a full graphical
environment often stay that way), this seriously impacts the usability
of qemu-system.

(Observed in QEMU 2.8)