docs: xl.cfg: permissive option is not PV only.

docs: xl.cfg: permissive option is not PV only.

Since XSA-131 qemu-xen has defaulted to non-permissive mode and the
option was extended to cover that case in 015a373351e5 "tools: libxl:
allow permissive qemu-upstream pci passthrough".

Since I was rewrapping to adjust the text anyway I've split the safety
warning into a separate paragraph to make it more obvious.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Cc: Eric <epretorious@yahoo.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
(cherry picked from commit 7f25baba1c0942e50757f4ecb233202dbbc057b9)