fix(vpp_device): pci address existence condition

The function get_pci_addr could check a different PCI_ADDR in case the
netdev device directory doesn't exit. Move the check so that it always
uses the proper PCI_ADDR and add a failure case when the netdev device
directory doesn't exist.

Change-Id: Ibadfea3fba34509cd3ea8f906ac5962c0f01dd17
Signed-off-by: Juraj Linkeš <juraj.linkes@pantheon.tech>