rust: pl011: pull interrupt updates out of read/write ops

rust: pl011: pull interrupt updates out of read/write ops

qemu_irqs are not part of the vmstate, therefore they will remain in
PL011State.  Update them if needed after regs_read()/regs_write().

Apply #[must_use] to functions that return whether the interrupt state
could have changed, so that it's harder to forget the call to update().

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