check if /proc/bus/pci/devices is available
check if /proc/bus/pci/devices is available

This avoids an error thrown if the file is not available.
Hint: the facter code is executed even if the module is
not included on the catalog.

Change-Id: Ie7a3dce6d42d3b2274bb5578862d677841e7bdc3

openstack/puppet-vswitch,openstack/puppet-vswitch