fix(Xapi#_assertHealthyVdiChain): fix for NFS SR prior XS 7.1 (#568)
fix(Xapi#_assertHealthyVdiChain): fix for NFS SR prior XS 7.1 (#568)

Fixes vatesfr/xo-web#2207
vatesfr/xo-web,lmcro/xo-web,lmcro/xo-web,lmcro/xo-web,vatesfr/xo-web