Adds StoragePool(), StoragePoolRefresh() (#28)

This adds two new methods:
- `StoragePool()`, used to lookup a storage pool by name.
- `StoragePoolRefresh()`, used to refresh a storage pool by name.

internal/constants/constants.go

@@ -42,6 +42,8 @@ const (
 	ProcDomainGetXMLDesc           = 14
 	ProcDomainLookupByName         = 23
 	ProcAuthList                   = 66
+	ProcStoragePoolRefresh         = 83
+	ProcStoragePoolLookupByName    = 84
 	ProcConnectGetLibVersion       = 157
 	ProcDomainMigrateSetMaxSpeed   = 207
 	ProcDomainGetState             = 212