Basing the cpu_index on the number of currently allocated vCPUs fails
when vCPUs aren't removed in a LIFO manner. This is especially true
when we are allocating a cpu_index for each guest thread in
linux-user where there is no ordering constraint on their allocation
and de-allocation.

[I've dropped the assert which is there to guard against out-of-order
removal as this should probably be caught higher up the stack. Maybe
we could just ifdef CONFIG_SOFTTMU it?]

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Acked-by: Igor Mammedow <imammedo@redhat.com>
Cc: Nikolay Igotti <igotti@gmail.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Eduardo Habkost <ehabkost@redhat.com>
Message-Id: <20200520140541.30256-13-alex.bennee@linaro.org>