Temporary fix for guest idle detection.

Handle ExtINT injection for SVM. The HPET emulation
will inject a legacy interrupt at startup, and if this
isn't handled, will result in the HLT-exit code assuming
there are outstanding ExtINTs and return without sleeping.

svm_inj_interrupts() needs more changes to bring it up
to date with the VT-x version: these are forthcoming.

Reviewed by: neel