KVM: Use thread debug register storage instead of kvm specific data

Instead of saving the debug registers from the processor to a kvm data
structure, rely in the debug registers stored in the thread structure.
This allows us not to save dr6 and dr7.

Reduces lightweight vmexit cost by 350 cycles, or 11 percent.

Signed-off-by: Avi Kivity <avi@redhat.com>