vl: initialize displays before preconfig loop

Displays should be available before the monitor starts, so that
it is possible to use the graphical console to interact with
the monitor itself.

This patch is quite ugly, but all this is temporary.  The double
call to qemu_init_displays will go away as soon we can unify machine
initialization between the preconfig and "normal" flows, and so will
the preconfig_exit_requested variable (that is only preconfig_requested
remains).

Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>