x86/traps: Clean up diagnostics

x86/traps: Clean up diagnostics

do{_unhandled,}_trap() should use fatal_trap() rather than opencoding part of
it.  This lets the remote stack trace logic work in more fatal error
conditions.

With do_trap() converted, there is only one single user of trapstr()
remaining.  Tweak the formatting in pv_inject_event(), and remove trapstr()
entirely.  Rename vec_name() to vector_name() now that it is exported.

Take the opportunity of vector_name() being exported to improve the
diagnostics in stub_selftest().

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>