x86: correct create_bounce_frame

x86: correct create_bounce_frame

Commit d9b7ef209a7 ("x86: drop failsafe callback invocation from
assembly") didn't go quite far enough with the cleanup it did: The
changed maximum frame size should also have been reflected in the early
address range check (which has now been pointed out to have been wrong
anyway, using 60 instead of 0x60), and it should have updated the
comment ahead of the function.

Also adjust the lower bound - all is fine (for our purposes) if the
initial guest kernel stack pointer points right at the hypervisor base
address, as only memory _below_ that address is going to be written.

Additionally limit the number of times %rsi is being adjusted to what
is really needed.

Finally move exception fixup code into the designated .fixup section
and macroize the stores to guest stack.

Reported-by: Jann Horn <jannh@google.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Release-acked-by: Julien grall <julien.grall@arm.com>