support optical drives with dev node /dev/sr1

Extend a prior fix which helped discovery of media on systems using 2.6
kernels.  /dev/sr0 covers only some of the use cases, /dev/sr1 is also
common.
DataSourceOVF: small fix for iso9660 transport

This is not really a problem, because nothing would call transport_iso9660
with 'require_iso' as False, but if it did, then we would have still
required iso9660 filesystem on the mount.
ConfigDrive, Nocloud: Apply the "poke /dev/sr0" approach to /dev/sr1 also

Optical drives may show up in RHEL systems as /dev/sr1;
this extends a prior fix to handle that use case. It's applicable to systems
with 2.6 kernels where blkid needs a little prodding.

If we have to go back here with '/dev/sr2', then we should probably come
up with a fix that doesn't doesn't make us come back again.