virpci: introduce virPCIDeviceExists()

virpci: introduce virPCIDeviceExists()

We're going to add logic to handle the case where a previously
existing PCI device does not longer exist in the host.

The logic was copied from virPCIDeviceNew(), which verifies if a
PCI device exists in the host, returning NULL and throwing an
error if it doesn't. The NULL is used for other errors as well
(product/vendor id read errors, dev id overflow), meaning that we
can't re-use virPCIDeviceNew() for the purpose of detecting
if the device exists.

Reviewed-by: Laine Stump <laine@redhat.com>
Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>