xenbus: add two more mb()s

Signed-off-by: Wei Liu <liuw@liuw.name>

xenbus: notify the other end when necessary

The xenbus thread didn't send notification to other end when it expected
more data or consumed responses, which led to stalling the ring from
time to time.

Fix this by sending notification to the other end at the right places.

Signed-off-by: Wei Liu <liuw@liuw.name>

Start introducing memory allocation arenas to bmk_memalloc.

Step 1: include information on who is allocation memory
in the memory allocation requests.

kill (hopefully) last libc dependency from platform/xen

use bmk_snprintf/vsnprintf() instead of sprintf()/vsprintf()

add bounds-checking to one call while doing so

use bmk_strtoul() instead of one leaked from libc

Add a minios_wait() macro.

It's for symmetry with minios_add_waiter(), so that we don't have
go editing a billion sites if we want to change bmk_sched().

xen/events: move notify_remote_via_evtchn to minios namespace

Signed-off-by: Wei Liu <liuw@liuw.name>

remove now-empty <mini-os/sched.h>

use bmk-core instead of stdio

change xen platform to use the scheduler interface from bmk-core

Reduce gratuitous differences between xen/baremetal sched.c

They are almost equivalent now, except for:

* clock/time routines
* irq save/restore (only in xen)
* naming differences (easy to fix)

Hide xen "struct thread" from the world.

This again brings the baremetal and xen schedulers closer together.

sensible layering drama, act 4

Reduce use of libc interfaces from the xen platform "kernel"
even further.

(what happened to act 3, I wonder ...)

convert platform/xen to rumprun_core memalloc

Move files into platform/xen

Prepares for the rumprun merge.