conf: Expose virDomainSCSIDriveAddressIsUsed
    
    This function checks if given drive address is already present in
    passed domain definition. Expose the function as it will be used
    shortly.
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
    Tested-by: Daniel Henrique Barboza <danielhb413@gmail.com>
    Reviewed-by: Jim Fehlig <jfehlig@suse.com>