loader.efi: Fix off by one error in size of the virtual disk

[imp]

Summary

The end address is the final byte in the array, not one byte past the
end of the array, so we need to add 1 to get the full length.

Fixes: 59219fc76a4b ("loader.efi: efiblk_memdisk_preload passes the VirtualDisks to FreeBSD")
Sponsored by: Netflix