KVM: use KVM_{GET|SET}_SREGS2 when supported.

This allows to make PDPTRs part of the migration
stream and thus not reload them after migration which
is against X86 spec.

Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
Message-Id: <20211101132300.192584-2-mlevitsk@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>