virpci: Resolve coverity issues

virpci: Resolve coverity issues

Coverity complains about "USE_AFTER_FREE" due to how virPCIDeviceSetStubDriver
"could" return either -1, 0, or 1 from the VIR_STRDUP() and then possibly makes
a call to virPCIDeviceDetach().

The only way this could happen is if NULL were passed as the "driver" name
and virStrdup() returned 0.  Since the calling functions check < 0 on the
initial function call, the 0 possibility causes Coverity to complain.

To fix this - enforce that the second parameter is not NULL using
ATTRIBUTE_NONNULL(2) for the function prototype, then in virPCIDeviceDetach
add an sa_assert(dev->stubDriver). This will result in Coverity not complaining
any more.