KVM: move DR register access handling into generic code

Currently both SVM and VMX have their own DR handling code. Move it to
x86.c.

Acked-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Gleb Natapov <gleb@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>