iotests: Avoid realpath, for CentOS 6

iotests: Avoid realpath, for CentOS 6

CentOS 6 lacks a realpath binary on the base install, which makes
all iotests runs fail since the 2.11 release:

001         - output mismatch (see 001.out.bad)
./check: line 815: realpath: command not found
diff: missing operand after `/home/dummy/qemu/tests/qemu-iotests/001.out'
diff: Try `diff --help' for more information.

Many of the uses of 'realpath' in the check script were being
used on the output of 'type -p' - but that is already an
absolute file name.  While a canonical name can often be
shorter (realpath gets rid of /../), it can also be longer (due
to symlink expansion); and we really don't care if the name is
canonical, merely that it was an executable file with an
absolute path.  These were broken in commit cceaf1db.

The remaining use of realpath was to convert a possibly relative
filename into an absolute one before calling diff to make it
easier to copy-and-paste the filename for moving the .bad file
into place as the new reference file even when running iotests
out-of-tree (see commit 93e53fb6), but $PWD can achieve the same
purpose.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Jeff Cody <jcody@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>