util: virdaemon: fix compilation on mingw

The daemons are not supported on Win32 and therefore were not compiled
in that platform. However, with the daemon code sharing, all the code in
utils *is* compiled and it failed because `waitpid`, `fork`, and
`setsid` are not available. So, as before, let's not build them on
Win32 and make the code more portable by using existing vir* wrappers.

Signed-off-by: Rafael Fonseca <r4f4rfs@gmail.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>