vlapic/viridian: abort existing APIC assist if any vector is pending in ISR

vlapic/viridian: abort existing APIC assist if any vector is pending in ISR

The vlapic code already aborts an APIC assist if an interrupt is deferred
because a higher priority interrupt has already been delivered (and hence
its vector is pending in the ISR).

However, it is also necessary to abort an APIC assist in the case where a
higher priority is about to be delivered because, in either case, at least
two vectors will be pending in the ISR and hence an EOI is necessary.

Also, following on from the above reasoning, the decision to start a new
APIC assist should clearly be based upon whether any other vector is
pending in the ISR, regardless of whether it is lower or higher in
priority. (In fact the code in question cannot be reached if the
vector is lower in priority). Thus the single use of
vlapic_find_lowest_vector() can be replaced with a call to
vlapic_find_highest_isr() and the former function removed.

Without this patch, because the logic is flawed, a domain_crash() results
when an attempt is made to erroneously start a new APIC assist.

Reported-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>