xen-pciback: simplify and tighten parsing of device IDs

Now that at least one of the conformance problems of the kernel's
sscanf() was addressed (commit da99075c),
we can improve the parsing done in xen-pciback both in terms of code
readability and correctness (in particular properly rejecting input
strings not well formed).

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>