It was found that a modified or corrupted image file can cause a DoS
on the host when getting image info with qemu-img.
This uses the newer 'prlimit' parameter for oslo.concurrency execute
to set an address space limit of 1GB and CPU time limit of 2 seconds
when running the qemu-img info command.
Change-Id: If5b7129b266ef065642bc7898ce9dcf93722a053
Closes-bug: #1449062
Sean McGinnis