qemu-iotests: Modern shell scripting (use $() instead of ``)

Various shell files contain a mix between obsolete ``
and modern $(); It would be nice to convert to using
$() everywhere.  For now, just do the qemu-iotests directory.

Cc: kwolf@redhat.com
Cc: mreitz@redhat.com
Cc: eblake@redhat.com
Suggested-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Mao Zhongyi <maozhongyi@cmss.chinamobile.com>
Message-Id: <20181024094051.4470-4-maozhongyi@cmss.chinamobile.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
[eblake: tweak commit message]
Signed-off-by: Eric Blake <eblake@redhat.com>
2 files changed