util: refactor storage file checks to allow error reporting

util: refactor storage file checks to allow error reporting

The virStorageFileSupportsSecurityDriver and
virStorageFileSupportsAccess currently just return a boolean
value. This is ok because they don't have any failure scenarios
but a subsequent patch is going to introduce potential failure
scenario. This changes their return type from a boolean to an
int with values -1, 0, 1.

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