docs: workaround markdown parser error in xen-command-line.markdown

docs: workaround markdown parser error in xen-command-line.markdown

Some versions of markdown (specifically the one in Debian Wheezy, currently
used to generate
http://xenbits.xen.org/docs/unstable/misc/xen-command-line.html) seem to be
confused by nested lists in the middle of multi-paragraph parent list entries
as seen in the com1,com2 entry.

The effect is that the "Default" section of all following entries are replace
by some sort of hash or checksum (at least, a string of 32 random seeming hex
digits).

Workaround this issue by making the decriptions of the DPS options a nested
list, moving the existing nested list describing the options for S into a third
level list. This seems to avoid the issue, and is arguably better formatting in
its own right (at least its not a regression IMHO)

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>