Use common objects for qemu-img and qemu-nbd

Use common objects for qemu-img and qemu-nbd

Right now, we sprinkle #if defined(QEMU_IMG) && defined(QEMU_NBD) all over the
code.  It's ugly and causes us to have to build multiple object files for
linking against qemu and the tools.

This patch introduces a new file, qemu-tool.c which contains enough for
qemu-img, qemu-nbd, and QEMU to all share the same objects.

This also required getting qemu-nbd to be a bit more Windows friendly.  I also
changed the Windows block-raw to use normal IO instead of overlapping IO since
we don't actually do AIO yet on Windows.  I changed the various #if 0's to
 #if WIN32_AIO to make it easier for someone to eventually fix AIO on Windows.

After this patch, there are no longer any #ifdef's related to qemu-img and
qemu-nbd.

Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@5226 c046a42c-6fe2-441c-8c8c-71466251a162