block: Rename BdrvChildRole to BdrvChildClass

block: Rename BdrvChildRole to BdrvChildClass

This structure nearly only contains parent callbacks for child state
changes.  It cannot really reflect a child's role, because different
roles may overlap (as we will see when real roles are introduced), and
because parents can have custom callbacks even when the child fulfills a
standard role.

Signed-off-by: Max Reitz <[email protected]>
Reviewed-by: Eric Blake <[email protected]>
Reviewed-by: Alberto Garcia <[email protected]>
Message-Id: <20200513110544[email protected]>
Signed-off-by: Kevin Wolf <[email protected]>