tools: hv: skip iso9660 mounts in hv_vss_daemon

fsreeze does not work for iso9660 filesystems. A ENOSUPP may be caught
in the freeze case, but the subsequent thaw call would fail and leads to
a false error.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>