tests/functional: make qemu commands specifiable

Current scripts under tests/functional use qemu-io and qemu-img
directly. This patch makes these commands specifiable via environment
variables: QEMU_IO_PROG and QEMU_IMG_PROG. This is important for
testing stable branches because output format of these commands can be
changed during update of qemu.

Example:
$ sudo QEMU_IO_PROG=/usr/local/qemu-stable-1.5/qemu-io DRIVER=local ./check 39

Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
tests/func: test restart with missing object in stale directory

Current stable-0.6 can't pass this script but master can.

Signed-off-by: Liu Yuan <namei.unix@gmail.com>
Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>