Fix build on non-Linux platforms

Fix build on non-Linux platforms

Commit ef48a1b introduced virFindSCSIHostByPCI for Linux and
a stub for other platforms that returns -1 while the function
should return 'char *', so use 'return NULL' instead.

Commit fbd91d4 introduced virReadSCSIUniqueId with the third
argument 'int *result', however the stub for non-Linux patform
uses 'unsigned int *result', so change it to 'int *result'.

Pushed under the build breaker rule.