x86/PV: split out dealing with DRn from privileged instruction handling

x86/PV: split out dealing with DRn from privileged instruction handling

This is in preparation for using the generic emulator here.

Some care is needed temporarily to not unduly alter guest register
state: The local variable "res" can only go away once this code got
fully switched over to using x86_emulate().

Also switch to IS_ERR_VALUE() instead of (incorrectly) open coding it.

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