hw/xen: Add emulated implementation of XenStore operations

hw/xen: Add emulated implementation of XenStore operations

Now that we have an internal implementation of XenStore, we can populate
the xenstore_backend_ops to allow PV backends to talk to it.

Watches can't be processed with immediate callbacks because that would
call back into XenBus code recursively. Defer them to a QEMUBH to be run
as appropriate from the main loop. We use a QEMUBH per XS handle, and it
walks all the watches (there shouldn't be many per handle) to fire any
which have pending events. We *could* have done it differently but this
allows us to use the same struct watch_event as we have for the guest
side, and keeps things relatively simple.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Reviewed-by: Paul Durrant <paul@xen.org>