configure: Suppress messages from pkg_config probe for check

Suppress confusing messages from pkg_config when probing for
'check' by sending them to /dev/null as we do with other
similar probes.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
