spapr: Fix RTAS token numbers

At the moment spapr_rtas_register() allocates a new token number for every
new RTAS callback so numbers are not fixed and depend on the number of
supported RTAS handlers and the exact order of spapr_rtas_register() calls.
These tokens are copied into the device tree and remain the same during
the guest lifetime.

When we start another guest to receive a migration, it calls
spapr_rtas_register() as well. If the number of RTAS handlers or their
order is different in QEMU on source and destination sides, the "/rtas"
node in the device tree will differ. Since migration overwrites the device
tree (as it overwrites the entire RAM), the actual RTAS config on
the destination side gets broken.

This defines global contant values for every RTAS token which QEMU
is using today.

This changes spapr_rtas_register() to accept a token number instead of
allocating one. This changes all users of spapr_rtas_register().

This changes XICS-KVM not to cache tokens registered with KVM as they
constant now.

This makes TOKEN_BASE global as RTAS_XXX use TOKEN_BASE as
a base. TOKEN_MAX is moved and renamed too and its value is changed
to the last token + 1. Boundary checks for token values are adjusted.

This reserves token numbers for "os-term" handlers and PCI hotplug
which we are working on.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Alexander Graf <agraf@suse.de>
