xen: fix invalid assertion

Asserting "true" is not that useful.

Reported by Coverity.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
