Move QEMU OS dependant library functions to OS specific files

Move QEMU OS dependant library functions to OS specific files

This moves library functions used by both QEMU and the QEMU tools,
such as qemu-img, qemu-nbd etc. from osdep.c to oslib-{posix,win32}.c

In addition it introduces oslib-obj.y to the Makefile set to be
included by the various targets, instead of relying on these library
functions magically getting included via block-obj-y.

Signed-off-by: Jes Sorensen <Jes.Sorensen@redhat.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>