qga: replace usleep() with g_usleep()

The latter simply requires glib.h, while the former is not in the
Windows API (but provided by mingw header & CRT)

Also simplify the expression for 1/10s.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20220420132624.2439741-12-marcandre.lureau@redhat.com>