Fix error handling in virSecurityManagerGetMountOptions

Fix error handling in virSecurityManagerGetMountOptions

The impls of virSecurityManagerGetMountOptions had no way to
return errors, since the code was treating 'NULL' as a success
value. This is somewhat pointless, since the calling code did
not want NULL in the first place and has to translate it into
the empty string "". So change the code so that the impls can
return "" directly, allowing use of NULL for error reporting
once again

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>