The SYSFS_PCI_DEVICES is a constant that makes the PCI testing
difficult as it points to an absolute path. We remove using this
constant and introducing a function pci_get_sysfs_path that gives
the same value. However, the user can pass a SYSFS_PCI_DEVICES env
variable to override the path. It is now possible to create a fake
sysfs hierarchy for testing.
Signed-off-by: Jan Viktorin <viktorin@rehivetech.com>