iotests: Don't check qemu_io() output for specific error strings

iotests: Don't check qemu_io() output for specific error strings

A forthcoming commit updates qemu_io() to raise an exception on non-zero
return by default, and changes its return type.

In preparation, simplify some calls to qemu_io() that assert that
specific error message strings do not appear in qemu-io's
output. Asserting that all of these calls return a status code of zero
will be a more robust way to guard against failure.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Hanna Reitz <hreitz@redhat.com>
Message-Id: <20220418211504.943969-4-jsnow@redhat.com>
Signed-off-by: Hanna Reitz <hreitz@redhat.com>