storage: fs: Ignore volumes that fail to open with EACCESS/EPERM

Trying to use qemu:///session to create a storage pool pointing at
/tmp will usually fail with something like:

$ virsh pool-start tmp
error: Failed to start pool tmp
error: cannot open volume '/tmp/systemd-private-c38cf0418d7a4734a66a8175996c384f-colord.service-kEyiTA': Permission denied

If any volume in an FS pool can't be opened by the daemon, the refresh
fails, and the pool can't be used.

This causes pain for virt-install/virt-manager though. Imaging a user
downloads a disk image to /tmp. virt-manager wants to import /tmp as
a storage pool, so we can detect what disk format it is, and set the
XML correctly. However this case will likely fail as explained above.

Change the logic here to skip volumes that fail to open. This could
conceivably cause user complaints along the lines of 'why doesn't
libvirt show $ROOT-OWNED-VOLUME-FOO', but figuring that currently
the pool won't even startup, I don't think there are any current
users that care about that case.

https://bugzilla.redhat.com/show_bug.cgi?id=1103308

src/storage/storage_backend.c

@@ -1445,6 +1445,10 @@ virStorageBackendVolOpen(const char *path, struct stat *sb,
             VIR_WARN("ignoring missing fifo '%s'", path);
             return -2;
         }
+        if ((errno == EACCES || errno == EPERM) && noerror) {
+            VIR_WARN("ignoring permission error for '%s'", path);
+            return -2;
+        }
 
         virReportSystemError(errno, _("cannot open volume '%s'"), path);
         return -1;