KVM: selftests: Force load all supported XSAVE state in state test

Extend x86's state to forcefully load *all* host-supported xfeatures by
modifying xstate_bv in the saved state.  Stuffing xstate_bv ensures that
the selftest is verifying KVM's full ABI regardless of whether or not the
guest code is successful in getting various xfeatures out of their INIT
state, e.g. see the disaster that is/was MPX.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20230928001956.924301-6-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>