Remove the spin on VCPUF_running from context_switch().

Remove the spin on VCPUF_running from context_switch().
It doesn't work because we already set VCPUF_running
locally, and the scheduler should not pick another CPU's
VCPUF_running VCPU anyway, since it mustn't modify the
processor field.

Instead just add a simple assertion to the scheduler to
ensure the scheduling algorithm is not picking an
inappropriate VCPU.

Signed-off-by: Keir Fraser <keir@xensource.com>