block: Add 'x-blockdev-del' QMP command

block: Add 'x-blockdev-del' QMP command

This command is still experimental, hence the name.

This is the companion to 'blockdev-add'. It allows deleting a
BlockBackend with its associated BlockDriverState tree, or a
BlockDriverState that is not attached to any backend.

In either case, the command fails if the reference count is greater
than 1 or the BlockDriverState has any parents.

Signed-off-by: Alberto Garcia <[email protected]>
Reviewed-by: Max Reitz <[email protected]>
Message-id: 6cfc148c77aca1da942b094d811bfa3fcf7ac7bb.1446475331[email protected]
Signed-off-by: Max Reitz <[email protected]>