xen: credit2: non Credit2 pCPUs are ok during shutdown/suspend.

xen: credit2: non Credit2 pCPUs are ok during shutdown/suspend.

Commit 7478ebe1602e6 ("xen: credit2: fix shutdown/suspend
when playing with cpupools"), while doing the right thing
for actual code, forgot to update the ASSERT()s accordingly,
in csched2_vcpu_migrate().

In fact, as stated there already, during shutdown/suspend,
we must allow a Credit2 vCPU to temporarily migrate to a
non Credit2 BSP, without any ASSERT() triggering.

Move them down, after the check for whether or not we are
shutting down, where the assumption that the pCPU must be
valid Credit2 ones, is valid.

Signed-off-by: Dario Faggioli <dario.faggioli@citrix.com>