qemu-iotests: fix pylint 2.8 consider-using-with error

qemu-iotests: fix pylint 2.8 consider-using-with error

pylint 2.8 introduces consider-using-with error, suggesting
to use the 'with' block statement when possible.

Modify all subprocess.Popen call to use the 'with' statement,
except the one in __init__ of QemuIoInteractive class, since
it is assigned to a class field and used in other methods.

Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
Message-Id: <20210510190449.65948-1-eesposit@redhat.com>
[mreitz: Disable bad-option-value warning in the iotests' pylintrc, so
         that disabling consider-using-with in QemuIoInteractive will
         not produce a warning in pre-2.8 pylint versions]
Signed-off-by: Max Reitz <mreitz@redhat.com>