util: Introduce vfio helpers

This is a library to manage the host vfio interface, which could be used
to implement userspace device driver code in QEMU such as NVMe or net
controllers.

Signed-off-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20180116060901.17413-3-famz@redhat.com>
Signed-off-by: Fam Zheng <famz@redhat.com>