fix xff excessive rounding

Hi,

When generating arguments for rrd_create, the previous
format string "%.1f", was rounding up the value too aggressively.

ex: for xff==0.9999 the result would be -> 1.0 (invalid value for xff!)

The new format string is "%.10f", which lead up to 10 digits after the
dot, enough to be close to the double type limit, and so handling
correctly the previous example.

Change-Id: I7c6cc55b0d43beb5ef351bb04b67f4628249c737