docs: add some missing options to misc/xen-command-line.markdown

docs: add some missing options to misc/xen-command-line.markdown

These were mostly ones from xen/arch/x86/boot/cmdline.S which are handled early
and therefore do not use the usual infrastructure and so got missed in the
initial trawl.

The document now contains (AFAICT) every still valid option which was
previously documented at:
http://wiki.xen.org/wiki?title=Xen_Hypervisor_Boot_Options&oldid=1379

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Committed-by: Ian Jackson <ian.jackson.citrix.com>