block: Add bitmap disabled status

block: Add bitmap disabled status

Add a status indicating the enabled/disabled state of the bitmap.
A bitmap is by default enabled, but you can lock the bitmap into
a read-only state by setting disabled = true.

A previous version of this patch added a QMP interface for changing
the state of the bitmap, but it has since been removed for now until
a use case emerges where this state must be revealed to the user.

The disabled state WILL be used internally for bitmap migration and
bitmap persistence.

Signed-off-by: Fam Zheng <[email protected]>
Signed-off-by: John Snow <[email protected]>
Reviewed-by: Max Reitz <[email protected]>
Reviewed-by: Stefan Hajnoczi <[email protected]>
Message-id: 1429314609[email protected]
Signed-off-by: Stefan Hajnoczi <[email protected]>
Signed-off-by: Kevin Wolf <[email protected]>