storage: Add default mount options for fs/netfs storage pools

storage: Add default mount options for fs/netfs storage pools

https://bugzilla.redhat.com/show_bug.cgi?id=1584663

Modify the command generation to add some default options to the
fs/netfs storage pools based on the OS type. For Linux, it'll be
the "nodev, nosuid, noexec". For FreeBSD, it'll be "nosuid, noexec".
For others, just leave the options alone.

Modify the storagepoolxml2argvtest to handle the fact that the
same input XML could generate different output XML based on whether
Linux, FreeBSD, or other was being built.

Signed-off-by: John Ferlan <jferlan@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>