qcow: require image size to be > 1 for new images

The qcow driver refuses to open images which are less than
2 bytes in size, but will happily create such images. Add
a check in the create path to avoid this discrepancy.

Reviewed-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Message-id: 20170623162419.26068-5-berrange@redhat.com
Signed-off-by: Max Reitz <mreitz@redhat.com>
