MAINTAINERS: Add a section for the host OS and a W32 maintainer

Up to now, there was no special section for the different
host operating systems used with QEMU.

scripts/get_maintainer.pl did not show a maintainer for
OS specific files and patches.

Therefore I added three hosts systems:

* POSIX for the majority of host systems which are supported.
  This includes BSD and Linux host systems.

* LINUX is a special case of POSIX needed for some Linux specific
  files and directories.

* W32, W64 for a well known family of closed source operating systems.

I also added myself as a maintainer for W32, W64.

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>