  [Abridged] We report on deep, coordinated radio and X-ray observations of the
black hole X-ray binary XTE J1118+480 in quiescence. The source was observed
with the Karl G. Jansky Very Large Array for a total of 17.5 hrs at 5.3 GHz,
yielding a 4.8 \pm 1.4 microJy radio source at a position consistent with the
binary system. At a distance of 1.7 kpc, this corresponds to an integrated
radio luminosity between 4-8E+25 erg/s, depending on the spectral index. This
is the lowest radio luminosity measured for any accreting black hole to date.
Simultaneous observations with the Chandra X-ray Telescope detected XTE
J1118+480 at 1.2E-14 erg/s/cm^2 (1-10 keV), corresponding to an Eddington ratio
of ~4E-9 for a 7.5 solar mass black hole. Combining these new measurements with
data from the 2005 and 2000 outbursts available in the literature, we find
evidence for a relationship of the form ellr=alpha+beta*ellx (where ell denotes
logarithmic luminosities), with beta=0.72\pm0.09. XTE J1118+480 is thus the
third system, together with GX339-4 and V404 Cyg, for which a tight, non-linear
radio/X-ray correlation has been reported over more than 5 dex in ellx. We then
perform a clustering and linear regression analysis on what is arguably the
most up-to-date collection of coordinated radio and X-ray luminosity
measurements from quiescent and hard state black hole X-ray binaries, including
24 systems. At variance with previous results, a two-cluster description is
statistically preferred only for random errors <=0.3 dex in both ellr and ellx,
a level which we argue can be easily reached when the known spectral
shape/distance uncertainties and intrinsic variability are accounted for. A
linear regression analysis performed on the whole data set returns a
best-fitting slope beta=0.61\pm0.03 and intrinsic scatter sigma_0=0.31\pm 0.03
dex.
