vfio: fix return type of pread size_t is an unsigned type, thus the error case is never reached in the below call to pread. If bytes is negative, it will be seen as a very high positive value. Spotted by Coverity. Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> Signed-off-by: Alex Williamson <alex.williamson@redhat.com>