iotests/migration-permissions: use assertRaises() for qemu_io() negative test

iotests/migration-permissions: use assertRaises() for qemu_io() negative test

Modify this test to use assertRaises for its negative testing of
qemu_io. If the exception raised does not match the one we tell it to
expect, we get *that* exception unhandled. If we get no exception, we
get a unittest assertion failure and the provided emsg printed to
screen.

If we get the CalledProcessError exception but the output is not what we
expect, we re-raise the original CalledProcessError.

Tidy.

(Note: Yes, you can reference "with" objects after that block ends; it
just means that ctx.__exit__(...) will have been called on it. It does
not *actually* go out of scope. unittests expects you to want to inspect
the Exception object, so they leave it defined post-exit.)

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Tested-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Hanna Reitz <hreitz@redhat.com>
Message-Id: <20220418211504.943969-9-jsnow@redhat.com>
Signed-off-by: Hanna Reitz <hreitz@redhat.com>