KVM: SVM: check for nested VINTR flag in svm_interrupt_allowed

Not checking for this flag breaks any nested hypervisor that does not
set VINTR. So fix it with this patch.

Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>
Signed-off-by: Avi Kivity <avi@redhat.com>