Simplify softmmu/main.c

Move qemu_main() declaration to a new header.

Simplify main.c since both cocoa & sdl cannot be enabled together.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Akihiko Odaki <akihiko.odaki@gmail.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20220420132624.2439741-5-marcandre.lureau@redhat.com>