KVM: clarify the idea of kvm_dirty_regs

This patch clarifies that kvm_dirty_regs are just a hint to the kernel and
that the kernel might just ignore some flags and sync the values (like done for
acrs and gprs now).

Signed-off-by: David Hildenbrand <dahi@linux.vnet.ibm.com>
Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>