ipmi: Add a UUID device property

Using the UUID that qemu generates probably isn't the best thing
to do, allow it to be passed in via properties, and use QemuUUID
for the type.

If the UUID is not set, return an unsupported command error.  This
way we are not providing an all-zero (or randomly generated) GUID
to the IPMI user.  This lets the host fall back to the other
method of using the get device id command to determind the BMC
being accessed.

Signed-off-by: Corey Minyard <cminyard@mvista.com>
Cc: Cédric Le Goater <clg@kaod.org>
Cc: David Gibson <david@gibson.dropbear.id.au>
Cc: Michael S. Tsirkin <mst@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>