valgrind/i386: avoid false positives on KVM_SET_XCRS ioctl

struct kvm_xcrs contains padding bytes. Let's use a designated
initializer to avoid false positives from valgrind/memcheck.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
