rust: pl011: only leave embedded object initialization in instance_init

rust: pl011: only leave embedded object initialization in instance_init

Leave IRQ and MMIO initialization to instance_post_init.  In Rust the
two callbacks are more distinct, because only instance_post_init has a
fully initialized object available.

While at it, add a wrapper for sysbus_init_mmio so that accesses to
the SysBusDevice correctly use shared references.

Reviewed-by: Zhao Liu <zhao1.liu@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>