KVM: Silence underflow warning in avic_get_physical_id_entry()

Smatch complains that we check cap the upper bound of "index" but don't
check for negatives.  It's a false positive because "index" is never
negative.  But it's also simple enough to make it unsigned which makes
the code easier to audit.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>