configure: Send error message from spice check to /dev/null

pkg-config is not always available (e.g. on win32 hosts),
but we don't want to see the 'command not found' error message.

Redirect stdout and stderr to /dev/null.

v2:

* Removed changes which should not have been here.

Cc: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Stefan Weil <weil@mail.berlios.de>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
