KVM: Fix off-by-one in overflow check of KVM_ASSIGN_SET_MSIX_NR

KVM_MAX_MSIX_PER_DEV implies that up to that many MSI-X entries can be
requested. But the kernel so far rejected already the upper limit.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Avi Kivity <avi@redhat.com>