vl: move icount configuration earlier

vl: move icount configuration earlier

Once qemu_tcg_configure is turned into a QOM property setter, it will not
be able to set a default value for mttcg_enabled.  Setting the default will
move to the TCG instance_init function, which currently runs before "-icount"
is processed.

However, it is harmless to do configure_icount for all accelerators; we will
just fail later if a non-TCG accelerator is selected.  So do that.

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