docs: Give advice on dealing with quoting special characters

docs: Give advice on dealing with quoting special characters

The man page mentions passing config snippets key=val in xl create.
Unfortunately, the config syntax contain characters which are
interpreted (and often discarded) by the shell before getting passed
in, resulting in a parsing error.

For example:

xl create hvm.cfg cpus="0-3"

The shell will eat the quotes and pass 'cpus=0-3' to xl, which won't
parse properly without the quotes.

Mention this in the man page, and recommend the use of single quotes, as well
as separating multiple arguments with a semicolon, thus:

xl create hvm.cfg 'cpus="0-3"; pci=["1.0","1.1"]'

Signed-off-by: George Dunlap <george.dunlap@eu.citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>