qemu: Add extra checks for secret destroy API's

qemu: Add extra checks for secret destroy API's

Remove the possibility that a NULL hostdev->privateData or a
disk->privateData could crash libvirtd by checking for NULL
before dereferencing for the secinfo structure in the
qemuDomainSecret{Disk|Hostdev}Destroy functions. The hostdevPriv
could be NULL if qemuProcessNetworkPrepareDevices adds a new
hostdev during virDomainNetGetActualHostdev that then gets
inserted via virDomainHostdevInsert. The hostdevPriv was added
by commit id '27726d8' and is currently only used by scsi hostdev.