memory: access FlatView from a local variable

memory: access FlatView from a local variable

We will soon require accesses to as->current_map to be placed under
a lock (with reference counting so as to keep the critical section
small).  To simplify this change, always fetch as->current_map into
a local variable and access it through that variable.

Reviewed-by: Anthony Liguori <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>