xen: fix XSA 462 description formatting

      In x86's APIC (Advanced Programmable Interrupt Controller) architecture,

      error conditions are reported in a status register.  Furthermore, the OS

      can opt to receive an interrupt when a new error occurs.

      It is possible to configure the error interrupt with an illegal vector,

      which generates an error when an error interrupt is raised.

      This case causes Xen to recurse through vlapic_error().  The recursion

      itself is bounded; errors accumulate in the the status register and only

      generate an interrupt when a new status bit becomes set.

      However, the lock protecting this state in Xen will try to be taken

      recursively, and deadlock.

    '';