exec.c: Allow target CPUs to define multiple AddressSpaces

exec.c: Allow target CPUs to define multiple AddressSpaces

Allow multiple calls to cpu_address_space_init(); each
call adds an entry to the cpu->ases array at the specified
index. It is up to the target-specific CPU code to actually use
these extra address spaces.

Since this multiple AddressSpace support won't work with
KVM, add an assertion to avoid confusing failures.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Acked-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>