block: Introduce op_blockers to BlockDriverState

BlockDriverState.op_blockers is an array of list with BLOCK_OP_TYPE_MAX
elements. Each list is a list of blockers of an operation type
(BlockOpType), that marks this BDS is currently blocked for certain type
of operation with reason errors stored in the list. The rule of usage
is:

 * BDS user who wants to take an operation should check if there's any
   blocker of the type with bdrv_op_is_blocked().

 * BDS user who wants to block certain types of operation, should call
   bdrv_op_block (or bdrv_op_block_all to block all types of operations,
   which is similar to bdrv_set_in_use of now).

 * A blocker is only referenced by op_blockers, so the lifecycle is
   managed by caller, and shouldn't be lost until unblock, so typically
   a caller does these:

   - Allocate a blocker with error_setg or similar, call bdrv_op_block()
     to block some operations.
   - Hold the blocker, do his job.
   - Unblock operations that it blocked, with the same reason pointer
     passed to bdrv_op_unblock().
   - Release the blocker with error_free().
block: Replace in_use with operation blocker

This drops BlockDriverState.in_use with op_blockers:

  - Call bdrv_op_block_all in place of bdrv_set_in_use(bs, 1).
  - Call bdrv_op_unblock_all in place of bdrv_set_in_use(bs, 0).
  - Check bdrv_op_is_blocked() in place of bdrv_in_use(bs).
    The specific types are used, e.g. in place of starting block backup,
    bdrv_op_is_blocked(bs, BLOCK_OP_TYPE_BACKUP, ...).
  - Check bdrv_op_blocker_is_empty() in place of assert(!bs->in_use).

Note: there is no block for only specific types for now, i.e. a caller
blocks all or none. So although the checks are type specific, the above
changes can still be seen as identical in logic.

Signed-off-by: Fam Zheng <famz@redhat.com>
block: Add checks of blocker in block operations

Before operate on a BlockDriverState, respective types are checked
against bs->op_blockers and it will error out if there's a blocker.

Signed-off-by: Fam Zheng <famz@redhat.com>
block: Parse "backing" option to reference existing BDS

Signed-off-by: Fam Zheng <famz@redhat.com>
qmp: add command 'blockdev-backup'

Similar to drive-backup, but this command uses a device id as target
instead of creating/opening an image file.

Also add blocker on target bs, since the target is also a named device
now.

Signed-off-by: Fam Zheng <famz@redhat.com>
block: Allow backup on referenced named BlockDriverState

Drive backup is a read only operation on source bs. We want to allow
this specific case to enable image-fleecing. Note that when
image-fleecing job starts, the job still add its blocker to source bs,
and any other operation on it will be blocked by that.

Signed-off-by: Fam Zheng <famz@redhat.com>