virDomainSetBlockThreshold: Clarify values of @dev the event is fired for
virDomainSetBlockThreshold: Clarify values of @dev the event is fired for

Top level image may get two events, one with the disk target (vda) and
one with disk target with index (vda[3]) if the top level image has an
index.

Signed-off-by: Peter Krempa <2cf5c04c61aa466e4a47bfedc747d17279c72ffc@redhat.com>
Reviewed-by: Eric Blake <a9da18479c871fe903d9590822cfe2629c6a4d11@redhat.com>

jardasgit/libvirt,zippy2/libvirt,zippy2/libvirt,libvirt/libvirt,jfehlig/libvirt,nertpinx/libvirt,crobinso/libvirt,olafhering/libvirt,olafhering/libvirt,crobinso/libvirt,jardasgit/libvirt,jardasgit/libvirt,jfehlig/libvirt,jfehlig/libvirt,nertpinx/libvirt,nertpinx/libvirt,nertpinx/libvirt,zippy2/libvirt,olafhering/libvirt,crobinso/libvirt,nertpinx/libvirt,zippy2/libvirt,jfehlig/libvirt,libvirt/libvirt,libvirt/libvirt,olafhering/libvirt,libvirt/libvirt,jardasgit/libvirt,jardasgit/libvirt,crobinso/libvirt