vscclient: use glib thread primitives not qemu

Use glib-provided thread primitives in vscclient instead of
qemu ones, and do not use qemu sockets in there (open-code
call to WSAStartup() for windows to initialize things).

This way, vscclient becomes more stand-alone, independent on
qemu internals.

Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
Reviewed-by: Alon Levy <alevy@redhat.com>
Tested-by: Alon Levy <alevy@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
