fix #7860: api: backup info: skip jobs with invalid guest selection

A backup job in pool mode keeps referencing its pool after the pool got
deleted. Resolving the guests of such a job dies with "pool '<name>'
does not exist", and since get_included_vmids() did not catch this, the
whole /cluster/backup-info/not-backed-up call failed. The GUI then hides
the "Show guests without backup job" button, so users lose exactly the
overview that would tell them some guests are no longer backed up.

Catch the error per job, warn about it and continue with the remaining
jobs. The broken job does not back up anything, so its guests correctly
show up as not covered by any backup job.

Add a test for get_included_vmids() covering this case.

Signed-off-by: Michal Fox <me@dualfroz.com>

test/Makefile

@@ -18,7 +18,7 @@ replication%.t: replication_test%.pl
 	./$<
 
 .PHONY: test-vzdump
-test-vzdump: test-vzdump-guest-included test-vzdump-new
+test-vzdump: test-vzdump-guest-included test-vzdump-new test-vzdump-backup-info
 
 .PHONY: test-vzdump-guest-included
 test-vzdump-guest-included:
@@ -28,6 +28,10 @@ test-vzdump-guest-included:
 test-vzdump-new:
 	./vzdump_new_test.pl
 
+.PHONY: test-vzdump-backup-info
+test-vzdump-backup-info:
+	./backup_info_test.pl
+
 .PHONY: test-osd
 test-osd:
 	./OSD_test.pl