vfio: Fix overrun after readlink() fills buffer completely
readlink() returns the number of bytes written to the buffer, and it
doesn't write a terminating null byte. vfio_init() writes it itself.
Overruns the buffer when readlink() filled it completely.
Fix by treating readlink() filling the buffer completely as error,
like we do in pci-assign.c's assign_failed_examine().
Spotted by Coverity.
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>