[dpdk-dev] eal/pci: Improve automatic selection of IOVA mode

[Ben Walker]

In SPDK, not all drivers are registered with DPDK at start up time.
Previously, that meant DPDK always chose to set itself up in IOVA_PA
mode. Instead, when the correct iova choice is unclear based on the
devices and drivers known to DPDK at start up time, use other heuristics
(such as whether /proc/self/pagemap is accessible) to make a better
choice.

This enables SPDK to run as an unprivileged user again without requiring
users to explicitly set the iova mode on the command line.