The sdl and gtk display options support more parameters than currently
documented. Also the "vnc" option got lost during a recent commit,
add it again.

Fixes: ddc71758 ("Add display suboptions to man pages")
Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20210630163231.467987-5-thuth@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>