KVM: SVM: use kvm_register_write()/read()

KVM has nice wrappers to access the register values, clean up a few places
that should use them but currently do not.

Signed-off-by: David Kaplan <david.kaplan@amd.com>
[forward port and testing]
Signed-off-by: Joel Schopp <joel.schopp@amd.com>
Acked-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>