Add utility function for finding utility binaries favoring built ones
It's a common test pattern to need to use QEMU binaries such as qemu-img, qemu-io, etc. It's a good idea to use the built ones, if available, but it's also usually OK to resort to those installed elsewhere in the system.
This pattern is used in boot_linux.BootLinuxBase.download_boot()
and should be abstracted and made available to other test writers.