./configure: request pkg-config to provide private libs when static linking

Added wrapper around pkg-config to allow:
- safe options injection via ${QEMU_PKG_CONFIG_FLAGS}
- spaces in path to pkg-config

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