hw/i386/pc: Use e820_get_num_entries() to access e820_entries

To be able to extract the e820* code out of this file (in the next
patch), access e820_entries with its correct helper.

Reviewed-by: Li Qiang <liq3ea@gmail.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20190818225414.22590-2-philmd@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>