Escape curly brackets inside single quotes (')

Escape curly brackets inside single quotes (')

text in backquotes (`) is not subject to asciidoc expansion but
text inside single quotes (') is

this behaviour led that '{vmid}' was somehow interpreted by asciidoc,
and the whole line was not not rendered

pveum.adoc was the only file with that problem as curly brackets
are inside backticks in the rest of the documentation