x86emul: correct EFLAGS.TF handling

x86emul: correct EFLAGS.TF handling

For repeated string instructions we should not emulate multiple
iterations in one go when a single step trap needs injecting (which
needs to happen after every iteration).

For all non-branch instructions as well as not taken conditional
branches we additionally need to take DebugCtl.BTF into consideration.

For mov-to/pop-into %ss there should be no #DB at all (EFLAGS.TF
remaining set means there'll be #DB after the next instruction).

Additionally retire.sti should remain clear when retire.singlestep gets
set to true.

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