maint: convert leading TABs in *.rng files to equivalent spaces

maint: convert leading TABs in *.rng files to equivalent spaces

* docs/schemas/capability.rng: Likewise.
* docs/schemas/network.rng: Likewise.
* docs/schemas/nodedev.rng: Likewise.
* docs/schemas/storagepool.rng: Likewise.
* docs/schemas/storagevol.rng: Likewise.
Use these commands:
t=$'\t'
git ls-files | grep '\.rng$' | xargs grep -lE "^ *$t" \
  | xargs perl -MText::Tabs -ni -le \
    '$m=/^( *\t[ \t]*)(.*)/; print $m ? expand($1) . $2 : $_'