doc: Add a page to document vstart.sh script

Signed-off-by: Christophe Courtaut <christophe.courtaut@gmail.com>
doc: Fix env variables in vstart.sh documentation

Signed-off-by: Christophe Courtaut <christophe.courtaut@gmail.com>
Merge pull request #395 from kri5/wip-vstart-documentation

doc: Add a page to document vstart.sh script