rust: add bindings for interrupt sources

rust: add bindings for interrupt sources

The InterruptSource bindings let us call qemu_set_irq() and sysbus_init_irq()
as safe code.

Interrupt sources, qemu_irq in C code, are pointers to IRQState objects.
They are QOM link properties and can be written to outside the control
of the device (i.e. from a shared reference); therefore they must be
interior-mutable in Rust.  Since thread-safety is provided by the BQL,
what we want here is the newly-introduced BqlCell.  A pointer to the
contents of the BqlCell (an IRQState**, or equivalently qemu_irq*)
is then passed to the C sysbus_init_irq function.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>