p2v: Remove --fullscreen option.

It did nothing and was left over from an earlier attempt to add
fullscreen mode.  That is not needed any longer since we now run
virt-p2v under the matchbox window manager.
p2v: gui: Get the correct button for cancel_button.

I was fetching cancel_button from the wrong dialog (conv_dlg, not
run_dlg), and therefore getting the wrong button.

This explains why the back button on the conversion dialog was always
insensitive.  It's because cancel_button was assigned to this button,
and then I was setting cancel_button to insensitive.
p2v: Add 'Reboot' button to the GUI (RHBZ#1165564).
p2v: Disable "Cancel Conversion" button after the conversion (RHBZ#1165569).

Actually this bug does nothing and remains cancelled all the
way through.  The next commit makes it function.
p2v: Make the "Cancel Conversion" button work (RHBZ#1165569).

This relies on the remote to keep sending us data.  If it hangs, then
the cancel button won't work.  This could also be fixed by introducing
a timeout to the read syscall.