qemu: block: Introduce helper function to ensure that block nodes are active

qemu: block: Introduce helper function to ensure that block nodes are active

Upcoming changes to snapshot code will break the assumption that block
nodes are always active (if the function is able to acquire a modify
job).

Introduce qemuBlockNodesEnsureActive that checks if the block graph in
qemu contains any inactive nodes and if yes reactivates everything.

The function will be used on code paths such as blockjobs which require
the nodes to be active.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>