Fix misc Win32 compile warnings

GCC >= 4.4 assumes the 'printf' attribute refers to the native
runtime libraries format specifiers. Thanks to gnulib, libvirt
has GNU format specifiers everywhere.  This means we need to
use 'gnu_printf' with GCC >= 4.4 to get correct compiler
checking of printf format specifiers.

* HACKING: Document new rules for ATTRIBUTE_FMT_PRINTF
* autobuild.sh, mingw32-libvirt.spec.in: Disable OpenNebula
  driver on mingw32 builds
* qemud/dispatch.h, qemud/qemu.h, src/buf.h src/internal.h,
  src/logging.h, src/security.h, src/sexpr.h, src/util.h,
  src/virterror_internal.h, src/xend_internal.c: Change
  over to ATTRIBUTE_FMT_PRINTF.
* src/virsh.c: Disable 'cd' and 'pwd' commands on Win32
  since they don't compile
* src/threads-win32.c: Add missing return value check

HACKING

@@ -312,7 +312,7 @@ gcc's printf attribute directive in the prototype.  For example, here's
 the one for virAsprintf, in util.h:
 
     int virAsprintf(char **strp, const char *fmt, ...)
-	ATTRIBUTE_FORMAT(printf, 2, 3);
+	ATTRIBUTE_FMT_PRINTF(2, 3);
 
 This makes it so gcc's -Wformat and -Wformat-security options can do
 their jobs and cross-check format strings with the number and types