basic/fileio: optimize buffer sizes in read_full_virtual_file()

basic/fileio: optimize buffer sizes in read_full_virtual_file()

We'd proceed rather inefficiently: the initial buffer size was LINE_MAX/2,
i.e. only 1k. We can read 4k at the same cost.

Also, we'd try to allocate 1025, 2049, 4097 bytes, i.e. always one higher than
the power-of-two size. Effectively the allocation would be bigger, and we'd
waste the additional space. So let's allocate aligned to the power-of-two size.
size=4095, 8191, 16383, so we allocate 4k, 8k, 16k.