iommu/vt-d: Adjust the return value of the parse_ioapics_under_ir

Adjust the return value of parse_ioapics_under_ir as
negative value representing failure and "0" representing
succcess. Just make it consistent with other function
implementations, and we can judge if calling is successfull
by if (!parse_ioapics_under_ir()) style.

Signed-off-by: Baoquan He <bhe@redhat.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>