x86/ioemul: Rewrite stub generation to be shadow stack compatible

x86/ioemul: Rewrite stub generation to be shadow stack compatible

The logic is completely undocumented and almost impossible to follow.  It
actually uses return oriented programming.  Rewrite it to conform to more
normal call mechanics, and leave a big comment explaining thing.  As well as
the code being easier to follow, it will execute faster as it isn't fighting
the branch predictor.

Move the ioemul_handle_quirk() function pointer from traps.c to
ioport_emulate.c.  There is no reason for it to be in neither of the two
translation units which use it.  Alter the behaviour to return the number of
bytes written into the stub.

Introduce a new nocall annotation using __attribute__((error)) to prohibit
calls being made.

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