acpi: fix file size check with -acpitable.

acpi table file can be modified during load so file size check
should be more strict.
pointer calculation should be after qemu_realloc(). not before realloc().

Signed-off-by: Isaku Yamahata <yamahata@valinux.co.jp>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
