KVM: Replaces symbolic permissions with numeric

Replaces "S_IRUGO | S_IWUSR" with 0644. The reason is that symbolic
permissions considered harmful:
https://lwn.net/Articles/696229/



Signed-off-by: Roman Storozhenko <romeusmeister@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>