memory: keep the owner of the AddressSpace alive until do_address_space_destroy

This fixes a use-after-free if do_address_space_destroy is executed
too late.

Reviewed-by: Michael Roth <mdroth@linux.vnet.ibm.com>
Tested-by: Michael Roth <mdroth@linux.vnet.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
