x86/svm Fixes and cleanup to svm_inject_event()

x86/svm Fixes and cleanup to svm_inject_event()

 * State adjustments (and debug tracing) for #DB/#BP/#PF should not be done
   for `int $n` instructions.  Updates to %cr2 occur even if the exception
   combines to #DF.
 * Don't opencode DR_STEP when updating %dr6.
 * Simplify the logic for calling svm_emul_swint_injection() as in the common
   case, every condition needs checking.
 * Fix comments which have become stale as code has moved between components.

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