xl: Remove global domid and enable -Wshadow

xl: Remove global domid and enable -Wshadow

Lots of functions loop over a list of domain and others take a domid as
a parameter, shadowing the global one and leading to all sorts of
confusion.

Therefore remove the global domid and explicitly pass it around as
necessary.

Adds a domid to the parameters for many functions and switches many
others from taking a char * domain specifier to taking a domid, pushing
the domid lookup to the toplevel.

Replaces some open-coded domain_qualifier_to_domid error checking with
find_domain.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
[ ijc -- annotate find_domain() with warn_unused_result and fix the
         handful of errors. ]
Committed-by: Ian Campbell <ian.campbell@citrix.com>