Build the array of command line arguments coming from config_host
once for all targets.  Add all accelerators to accel/Kconfig so
that the command line arguments for accelerators can be computed
easily in the existing "foreach sym: accelerators" loop.

Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>