memory: fix address space initialization/destruction

A couple of fields were left uninitialized.  This was not observed earlier
because all address spaces were statically allocated.  Also free allocation
for those fields.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Avi Kivity <avi.kivity@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
