vhost: fix vhost_get_avail_idx for a non empty ring

• azl3 kernel 6.6.157.1-1 on Azure Linux 3.0

• azl3 kernel 6.6.150.1-1 on Azure Linux 3.0

• azl3 kernel 6.6.152.1-1 on Azure Linux 3.0

SecurityAlert records field-level changes from the point we begin following a bulletin. Earlier vendor changes may not have a field-by-field record.

1. Sep 27, 2026 at 11:11 PM UTC
   • Added affected products: azl3 kernel 6.6.152.1-1 on Azure Linux 3.0, azl3 kernel 6.6.157.1-1 on Azure Linux 3.0.
   • The vendor changed its remediation guidance.

1. Aug 23, 2026 at 2:57 PM UTC
   Published by Microsoft Security Response Center

   The publication date reported by the vendor.

2. Aug 24, 2026 at 8:10 AM UTC
   Added to SecurityAlert

   We collected the advisory from the official source.

3. Sep 24, 2026 at 2:28 AM UTC
   Updated by Microsoft Security Response Center

   The vendor changed the advisory after it was first published.

4. Oct 28, 2026 at 3:45 AM UTC
   Confirmed at the source

   Our collector saw this advisory during a later source check.