fix(backend): warn if corrupt qcow file

Previously it died failing, now it complains but
carries on. Thanks to @alticlar for reporting

issue #1090

lib/Ravada/Domain/KVM.pm

@@ -1652,7 +1652,7 @@ sub _find_base {
     run3(\@cmd,\$in, \$out, \$err);
 
     my ($base) = $out =~ m{^backing file: (.*)}mi;
-    confess "No base for $file in $out" if !$base;
+    warn "No base for $file in $out" if !$base;
 
     return $base;
 }