x86: fix migration from pre-version 12

On KVM, the KVM_SET_XSAVE would be executed with a 0 xstate_bv,
and not restore anything.

Since FP and SSE data are always valid, set them in xstate_bv at reset
time.  In fact, that value is the same that KVM_GET_XSAVE returns on
pre-XSAVE hosts.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Gleb Natapov <gleb@redhat.com>
