KVM: Avoid spurious execeptions after setting registers

Clear pending exceptions when setting new register values. This avoids
spurious exceptions after restoring a vcpu state or after
reset-on-triple-fault.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Avi Kivity <avi@qumranet.com>