vl.c: Increase width of machine name column in "-M ?" output

vl.c: Increase width of machine name column in "-M ?" output

Increase the width of the column used for the machine name in
the "-M ?" output from 10 to 20 spaces. This fixes the formatting
so it looks nice for architectures where a few of the machines
have overly long names. (Our current longest machine name is
"petalogix-s3adsp1800" with "realview-eb-mpcore" not far behind.)

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>