qemu: Fix using guest architecture as lookup key

qemu: Fix using guest architecture as lookup key

When looking for a QEMU binary suitable for running ppc64le guests
we have to take into account the fact that we use the QEMU target
as key for the hash, so direct comparison is not good enough.

Factor out the logic from virQEMUCapsFindBinaryForArch() to a new
virQEMUCapsFindTarget() function and use that both when looking
for QEMU binaries available on the system and when looking up
QEMU capabilities later.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1260753