iotests/178: Pass value to invalid option

iotests/178: Pass value to invalid option

ccd3b3b8112 has deprecated short-hand boolean options (i.e., options
with values).  All options without values are interpreted as boolean
options, so this includes the invalid option "snapshot.foo" used in
iotest 178.

So after ccd3b3b8112, 178 fails with:

  +qemu-img: warning: short-form boolean option 'snapshot.foo' deprecated
  +Please use snapshot.foo=on instead

Suppress that deprecation warning by passing some value to it (it does
not matter which, because the option is invalid anyway).

Fixes: ccd3b3b8112b670fdccf8a392b8419b173ffccb4
       ("qemu-option: warn for short-form boolean options")
Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20210126123834.115915-1-mreitz@redhat.com>