xen: add CPU microcode update driver

xen: add CPU microcode update driver

Xen does all the hard work for us, including choosing the right update
method for this cpu type and actually doing it for all cpus.  We just
need to supply it with the firmware blob.

Because Xen updates all CPUs (and the kernel's virtual cpu numbers have
no fixed relationship with the underlying physical cpus), we only bother
doing anything for cpu "0".

[ Impact: allow CPU microcode update in Xen dom0 ]
Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>