virt-*-validate.in: quote all variable references

virt-*-validate.in: quote all variable references

Alas, the shell is not a real programming language.

Patch generated by manual confirmation of vim's
s/[^"]\@<=\$\S\+\s\@=/"&"/gc
and
s/\(echo \)\@<=[^"].*\$.*$/"&"/c matches.

This patch generate a lot of noise and carries little benefits, as
I do not really expect $PKI to contain spaces or backticks. I'm just
fuming, and would not really mind if this patch is ignored