x86/emul: Correct the handling of eflags with SYSCALL

x86/emul: Correct the handling of eflags with SYSCALL

A singlestep #DB is determined by the resulting eflags value from the
execution of SYSCALL, not the original eflags value.

By using the original eflags value, we negate the guest kernels attempt to
protect itself from a privilege escalation by masking TF.

(re)introduce a singlestep boolean, defaulting to the original eflags state,
but have the SYSCALL emulation recalculate it after masking has occurred.

This is XSA-204

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