qapi: Refine human printing of sizes

This fixes several bugs or shortcomings of the previous pretty-printer.
In particular:

* use PRIu64 instead of casting to long long

* the exact value is included too

* the correct unit of measure (MiB, GiB, etc.) is used.  PiB and EiB
are added too.

* due to an off-by-one error, 512*2^30 was printed as 0.500MiB rather than
512MiB.  floor(log2(val)) is equal to 63 - clz(val), while the code used 64.

* The desired specification is %g rather than %f, which always uses three
decimals in the current code.  However %g would switch to scientific
notation when the integer part is >= 1000 (e.g. 1000*2^30).  To keep the
code simple, switch to the higher power when the integer part is >= 1000;
overflow is avoided by using frexp instead of clz.

Suggested-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>
