KVM: apic round robin cleanup

If no apic is enabled in the bitmap of an interrupt delivery with delivery
mode of lowest priority, a warning should be reported rather than select
a fallback vcpu

Signed-off-by: Qing He <qing.he@intel.com>
Signed-off-by: Eddie (Yaozu) Dong <eddie.dong@intel.com>
Signed-off-by: Avi Kivity <avi@qumranet.com>