qemu-iotests: cleanup and fix search for programs

Instead of ./check failing when a binary is missing, we try each test
case now and each one fails with tons of test case diffs.  Also, all the
variables were initialized by "check" prior to "common" being sourced,
and then (uselessly) checked for emptiness again in "check".

Centralize the search for programs in "common" (which will soon be
one with "check"), including the "realpath" invocation which can be done
just once in "check" rather than in the tests.

For qnio_server, move the detection to "common", simplifying
set_prog_path to stop handling the unused second argument, and
embedding the "realpath" pass.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
