* Fix NPE in NASBackupProvider when no running KVM host is available
ResourceManager.findOneRandomRunningHostByHypervisor() can return null
when no KVM host in the zone has status=Up (e.g. during management
server startup, brief agent disconnections, or host state transitions).
NASBackupProvider.syncBackupStorageStats() and deleteBackup() call
host.getId() without a null check, causing a NullPointerException that
crashes the entire BackupSyncTask background job every sync interval.
This adds null checks in both methods:
- syncBackupStorageStats: log a warning and return early
- deleteBackup: throw CloudRuntimeException with a descriptive message