The string output visitor should serialize numbers so that the string
input visitor deserializes them back to the same number.  It fails to
do so.

print_type_number() uses format %f.  This is prone to nasty rounding
errors.  For instance, numbers between 0 and 0.0000005 get flushed to
zero.

We currently use this visitor only for HMP info migrate, info network,
info qtree, and info memdev.  No double values occur there as far as I
can tell.

Fix anyway by formatting with %.17g.  17 decimal digits always suffice
for IEEE double.

See also recent commit "qobject: Fix qnum_to_string() to use
sufficient precision".

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20201210161452.2813491-9-armbru@redhat.com>