iotests: Use absolute paths for executables

iotests: Use absolute paths for executables

A user may specify a relative path for accessing qemu, qemu-img, etc.
through environment variables ($QEMU_PROG and friends) or a symlink.

If a test decides to change its working directory, relative paths will
cease to work, however. Work around this by making all of the paths to
programs that should undergo testing absolute. Besides "realpath", we
also have to use "type -p" to support programs in $PATH.

As a side effect, this fixes specifying these programs as symlinks for
out-of-tree builds: Before, you would have to create two symlinks, one
in the build and one in the source tree (the first one for common.config
to find, the second one for the iotest to use). Now it is sufficient to
create one in the build tree because common.config will resolve it.

Reported-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-id: 20170702150510.23276-2-mreitz@redhat.com
Reviewed-by: Eric Blake <eblake@redhat.com>
Tested-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>