[libvirt] Expose qemu monitor command query_block?

[Osier Yang]

Command "query-block" is only used internally for medium's
checking before migration currently. Should we expose it
as a public API, such as virDomainQueryBlock (or a qemu
specific API, virDomainQemuQueryBlock)?

It has enough useful infomation which user may care out,
such as "io-status", "locked", "removable", "inserted". And
perhaps more info in future.

Osier