...
(XEN) microcode: collect_cpu_info : sig=0x40651, pf=0x40, rev=0x1c
(XEN) microcode: CPU2 found a matching microcode update with version 0x1f (current=0x1c)
(XEN) microcode: CPU2 updated from revision 0x1c to 0x1f, date = 2016-04-01
Note that in newer versions of Xen, many of the microcode messages are no longer traced out. So even though the microcode package is being loaded, if there is not microcode match for the system, nothing will be shown.