These accessors are called from inlined functions, and the call sequence
is much more expensive than just inlining the access.  Move the
struct declaration to memory-internal.h so that exec.c and memory.c
can both use an inline function.

Reviewed-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>