xen/x86: call hypercall handlers via generated macro

xen/x86: call hypercall handlers via generated macro

Instead of using a function table use the generated macros for calling
the appropriate hypercall handlers.

This is beneficial to performance and avoids speculation issues.

With calling the handlers using the correct number of parameters now
it is possible to do the parameter register clobbering in the NDEBUG
case after returning from the handler. With the additional generated
data the hard coded hypercall_args_table[] can be replaced by tables
using the generated number of parameters.

Note that this change modifies behavior of clobbering registers in a
minor way: in case a hypercall is returning -ENOSYS (or the unsigned
equivalent thereof) for any reason the parameter registers will no
longer be clobbered. This should be of no real concern, as those cases
ought to be extremely rare and reuse of the registers in those cases
seems rather far fetched.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>