Add stubdomain support. See stubdom/README for usage details.

Add stubdomain support. See stubdom/README for usage details.

- Move PAGE_SIZE and STACK_SIZE into __PAGE_SIZE and __STACK_SIZE in
  arch_limits.h so as to permit getting them from there without
  pulling all the internal Mini-OS defines.
- Setup a xen-elf cross-compilation environment in stubdom/cross-root
- Add a POSIX layer on top of Mini-OS by linking against the newlib C
  library and lwIP, and implementing the Unixish part in mini-os/lib/sys.c
- Cross-compile zlib and libpci too.
- Add an xs.h-compatible layer on top of Mini-OS' xenbus.
- Cross-compile libxc with an additional xc_minios.c and a few things
  disabled.
- Cross-compile ioemu with an additional block-vbd, but without sound,
  tpm and other details.  A few hacks are needed:
  - Align ide and scsi buffers at least on sector size to permit
  direct transmission to the block backend. While we are at it, just
  page-align it to possibly save a segment. Also, limit the scsi
  buffer size because of limitations of the block paravirtualization
  protocol.
  - Allocate big tables dynamically rather that letting them go to
  bss: when Mini-OS gets installed in memory, bss is not lazily
  allocated, and doing so during Mini-OS is unnecessarily trick while
  we can simply use malloc.
- Had to change the Mini-OS compilation somehow, so as to export
  Mini-OS compilation flags to the Makefiles of libxc and ioemu.

Signed-off-by: Samuel Thibault <samuel.thibault@eu.citrix.com>