qemu: Fix error message when PCI bridge has index <= bus

qemu: Fix error message when PCI bridge has index <= bus

Commit ff2126225df0 changed the error message to be more
detailed about the failure at hand; however, while the new
error message claims that "bus must be <= index", the error
message is displayed if "idx <= addr->bus", ie. when bus
is larger than or *equal to* index.

Change the error message to report the correct constraint,
and format it in a way that mirrors the check exactly to
make it clearer to people reading the code. The new error
message reads "index must be larger than bus".

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1339900