qemu: remove redundant check for file length when determining PCIe vs. PCI

qemu: remove redundant check for file length when determining PCIe vs. PCI

Now that virPCIDeviceIsPCIExpress() checks the length of the file when
the process lacks sufficient privilege to read the entire PCI config
file in sysfs, we can remove the open-coding for that case from its
consumer.

Signed-off-by: Laine Stump <laine@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>